The Dallas Cowboys' quest for another set of Super Bowl rings ended abruptly today on a stunning 50-yard scoring strike with 126 seconds left that gave the long-frustrated Los Angeles Rams some rare playoff happiness.

"The Cowboys' luck finally ran out," said defensive back Eddie Brown in a joyous Ram dressing room after Los Angeles had recorded its 21-19 upset to advance to the NFC title game Sunday in Tampa Bay.

"They were due," Brown said. "They've been beating people for years with big plays. Today they found out what it was like."

The Rams, long losers in playoff games they were favored to win, found out what it was like to celebrate an unexpected victory, thanks to that partially-deflected touchdown bomb from quarterback Vince Ferragamo to receiver Billy Waddy into the heart of the Cowboys' prevent defense.

This was a Dallas club that survived all season on last-minute combacks. Los Angeles, which stumbled into the playoffs from the weakest of all NFL divisions, throttled Roger Staubach with seven defensive backs and the kind of aggressiveness that belied the team's 9-7 regular season record.

Ferragamo, playing only because of a midseason injury to Pat Haden, came back from two early second-half interceptions to finish with three scoring passes and outshine the veteran Staubach.

All three of Ferragamo's TD throws were long. He tossed a 32-yarder to Wendell Tyler and a 43-yarder to Ron Smith with three seconds left in the opening half.

"They had three big plays," Dallas safety Randy Hughes said. "We just gave up the big pass play. It hurt us all year and it hurt us today."

Staubach had overcome such defensive weaknesses to pull out four games this year in the last two minutes. With their quarterback unable to solve the shifting coverages, the Cowboy defenders finally caved in. Dallas will not make its fourth trip in the last five years to the Super Bowl.

"We came and got our respect back," said Ram cornerback Pat Thomas. "This is so sweet. They've killed us for years on big plays. It was nice to do it to them."

This was an unlikely win for Los Angeles. The Rams' powerhouse teams in the past had failed in the playoffs against weaker opponents, yet here was this club, riddled all season by injuries and playing with an inexperienced quarterback in his sixth pro start, toppling the mighty Cowboys.

"Games like this make champions," said end Fred Dryer. "Or maybe champions make games like this. Everyone counted us out but we've been improving while no one has been watching us."

Until Waddy's touchdown catch, the Rams thought this one was being taken from them by some questionable officiating. Most controversial was a call midway through the fourth period when it appeared Dallas' Cliff Harris had interfered with Drew Hill on a fourth-down pass at the Cowboy 20.

A flag was thrown on the play but after a huddle by the officials, the penalty was canceled. According to referee Jerry Markbreit, the official who made the call (Ray Douglas) yeilded to field judge Charley Musser who felt there was no interference.

So the Cowboys took over on downs leading, 19-14, and Ram Coach Ray Malavasi was left to protest "It was a bad call, that's all there is to it. It was obvious it is pass interference."

Dallas was unable to sustain any offense and finally, with 2:16 left, the Rams had the ball after a short punt at the 50.

On first down, Ferragamo called a play designed for a pass to Smith on the left side. But Smith was blanketed by some of the Cowboys' eight pass defenders and Ferragamo looked toward Waddy, who was being covered by Benny Barnes.

"I would have had it easy but someone (Dallas linebacker Mike Hegman) tipped it slightly," said Waddy, "and I said, 'I ain't gonna let that son-of-a-gun get past me.'"

Waddy pulled in the pass, got a good block from Smith and found himself 30 yards from the Dallas end zone and no Cowboys in the way.

"It was a footrace and no one was going to catch me," he said. "I couldn't believe no one was near me."

Waddy sprinted into the end zone, Pat Corral added the PAT and with 2:06 on the clock, the Rams were ahead, 21-19.

The Cowboys had turned away defeat so many times this season that their fans probably expected another Staubach miracle. Hadn't he just pulled out a game against Washington two weeks ago in the last three minutes with his team trailing by 13 points?

This time, there were no heroics. After Tony Dorsett picked up 12 yards on first down, Staubach threw two incomplete passes and then tried to ground a third. Instead, it wound up in the stomach of guard Herb Scott, costing the Cowboys the down and 10 yards on an illegal-receiver penalty.

On fourth down, Staubach threw over the middle to Drew Pearson at midfield. "I had the time and laid it in there where I thought Drew could get it. It was a just a little high."

Pearson reached for the pass but couldn't pull it in. With 1:08 remaining, the Cowboy season was over.

"I don't think Roger ever knew who was covering who out there," Ram linebacker Jack Reynolds said. "He won't admit it but he didn't." Added Malavasi: "We knew we had to stop their two-minute drill. By putting seven backs out there, we were matching speed with speed. It was very effective."

The decision to use seven backs was made by Bud Carson, Ram Defensive coordinator. Its aim, according to Brown, was "to take away their passes to the halfback, especially to Preston Pearson. We've got nine defensive backs here and we have the speed to match their backs. It makes more sense than to have linebackers cover them."

Preston Pearson was limited to two catches for 15 yards while Staubach completed just 13 of 28 passes for 150 yards and one touchdown. Brown also intercepted one throw.

"They outfoxed us a few times in the first half," Staubach said. "You have to give them credit."

The Rams had taken a 14-5 halftime lead on the pass to Tyler. He had beaten linebacker D. D. Lewis ("They were attacking our defense where we were camouflaging," said Harris) and the bomb to Smith between two Dallas Defenders ("They just threw it up," said safety Randy Hughes, who couldn't raise one arm because of a shoulder separation).

Staubach, helped by some adjustments, got untracked somewhat in the third period.He connected with Drew Pearson for 29 yards to launch one drive, which was aided by a pass interference call on linebacker Jim Youngblood in the end zone. Ron Springs finally dove over from the one and the Cowboys trailed, 14-12.

Ferragamo then threw his second interception of the half. He tried to hit Waddy deep but Harris surprised the Ram Quarterback by stepping into the pattern and grabbing the pass, returning it to the Dallas 42.

An 11-yard completion to Preston Pearson and a 15-yarder to Jay Saldi had the ball on the Ram 24. Staubach found Drew Pearson on a slant-in for 15 more and a first down at the nine.

On a third down from the two, Dorsett went in motion to the right. The Ram defense shifted to cover him and Staubach faked that way before sending a bullet into Saldi's stomach in the end zone. There wasn't a Dallas Defender anywhere in sight.

"It was the same pass that we threw to Jackie Smith in the Super Bowl last Year," Saldi said. Smith dropped that toss; Saldi held on today.

Now the Cowboys, who had managed only a 33-yard Rafael Septien field goal and a safety (when Ferragamo slipped and fell into the end zone) in the first half, had a 19-14 lead with 12:46 left.

"No one panicked," Ferragamo said. "Even after that interfernce call, we thought we had a chance. If you write about me, better give everyone else credit too. This has been a long season for everyone."

Ferragamo had endured media criticism and the boos of the L.A. fans. The Rams endured those multiple injuries, the death of owner Carroll Rosenbloom and constant front office turmoil.

"We are a different team than we were six games ago or four games ago," said Dryer. "Considering what we have been through, you can't appreciate what this means. We may have had a sweeter win, but I can't remember when. And right now, I don't want to."