Washington's late-season drive for a playoff berth stalled tonight as the NASL's glamor boys, the Cosmos, found their old touch and punished the Diplomats, 4-2, at Giants Stadium.

The loss, Washington's third in four meetings with the Cosmos, left the Dips with a 14-16 record and 126 points. Washington and Portland are tied for 14th place and Jacksonville has 124 points. The top 15 teams in points advance to the playoffs. The Cosmos improved their league-best marks to 22-8 and 191 points.

Unlike past games, Washington was not done in by Giorgio Chinaglia. The NASL scoring leader did score a goal, his 29th of the season, but Roberto Cabanas was the hero in front of an appreciative crowd of 34,884. The 20-year-old winger, playing his first game in a month, didn't show any signs of the groin-muscle injury that took him out of the lineup. He scored a classic goal and assisted on the other three in a game Chinaglia said the team needed.

"We had lost three straight and maybe everyone felt a lot of pressure," said Chinaglia. "Had we lost tonight, I don't know what the hell would have happened to us. We had no offense the last few games. We had been waiting for the defense to give us a break and that's not our style."

The Cosmos returned to their old attacking ways tonight and Washington couldn't stop them. Even had injured Johan Cruff played, it's doubtful he would have made any difference.

"I wasn't impressed with them the first few games but tonight they proved they're one of the best sides, or the best, in the league," Dip defender Malcolm Waldron said. "We got down, 2-0, but it's no disgrace being down two goals to the best side. We gave away that goal before the half, but we always do that, don't we."

Washington trailed, 2-l, before allowing Johan Neeskens to chip in a header with five seconds left in the half. The Dips cut the deficit to 3-2 on Waldron's 15th goal of the year at 71:20 but the Cosmos weren't about to allow the visitors to tie the game.

At the 72:57 mark, reserve Chico Borja knifed in a shot to make it 4-2.

"That fourth goal put it out of reach," said Dip Coach Ken Furphy. "It was a brilliant goal."

Both teams scored goals that Cosmo defender Robert Iarusci said were unstoppable.

"Both teams had something to play for and maybe we put out a little extra," said Iarusci. "We played harder because we had to come out of this slump."

With Cabanas gaining confidence with each run downfield, the Cosmos' offense was almost indefensible. The team outshot Washington by only a goal, 16-15, but Dip goalie Jim Brown was forced to make some saves bordering on the imposssible to keep the game as close as it was.

Coming into tonight's contest, the Cosmos had lost three straight games, the most since 1974. As if that wasn't incentive enough, they had only to peek over their shoulders to see Chicago breathing down their necks in the race for the regular season crown and the playoff bye that goes with it.

The game was barely 10 minutes old when Chinaglia took a high cross from Julio Cesar Romero and headed in his team's first goal of the night. The textbook play began when Cabanas controlled a long ball and began a run upfield. He passed the ball to an all-alone Romero on the left side of the box and the forward relayed perfectly to Chinaglia.

That was only the beginning of what was to be a long, frustrating evening for the Dips. Almost 10 minutes later, Cabanas volleyed a rocket from 22 yards out past Brown to put the Cosmos up, 2-0, at 19:08.

The goal was unassisted although Dip defender Heinz Wirtz had a hand or head in the play. Cosmo sweeper Wim Rijsbergen sent the ball toward the Washington box and Wirtz leaped up in an attempt to cut it off. Unfortunately for the Dips, the carom dropped at the feet of Cabanas, who stroked it cleanly past Brown. Wirtz atoned by chipping a 12-yard shot past a surprised Cosmo keeper, Hubert Birkenmeier, at 20:29. The goal appeared to serve as further incentive for the home team.

On the Cosmos' sixth corner kick, Vladislav Bogicevic sent a pass to Cabanas, who had only to chip it into the mouth of the box, where four greedy, white-shirted Cosmos were waiting. With five seconds left, Neeskens outjumped teammates Chinaglia and Jeff Durgan for the ball and easily headed it inside the left post for a 3-1 lead at intermission.

The Cosmos took one less shot than Washington, six to five, in the half but three of those five attempts were right on target.

Washington midfielder David McGill suffered a broken bone and ruptured ligaments in his right ankle in the game and is probably out for the season.