So much for the New York Islanders' fond dream of an easy sweep of their Patrick Division final with the New York Rangers.

The Islanders, who had displayed playoff hockey to the max two nights before to take a 2-0 lead in the best-of-seven series, did a disappearing act for two periods tonight while the Rangers dominated the game end to end, taking a 7-2 lead into the third period before the Stanley Cup champions came back to play.

But the wakeup call came too late and the Rangers took home a 7-6 victory and a guarantee of at least two more games in their season. The teams play in Madison Square Garden Monday night and at Nassau Coliseum Wednesday.

"We got in trouble on penalties early," said Islanders Coach Al Arbour and, indeed, a pair of Rangers power play goals in the first period and another in the second left the visitors reeling in confusion.

The Islanders had opened the game with a power play score of their own, Mike Bossy's goal at 5:06 of the first period. But then Ken Morrow got two tripping penalties, and the Rangers capitalized on both, with goals by George McPhee and Eddie Johnstone.

Bob Bourne brought the score to 4-2 with his short-handed goal halfway through that period, but two more Rangers goals, one by Ron Duguay and a shot from the blueline by Barry Beck, gave the Islanders a glimpse of what was in store.

Starting goalie Billy Smith gave up another goal, this time to Dave Maloney, to open the second period, and was replaced by Rollie Melanson. But by then, the Rangers were indeed on a roll, scoring two more to swell their edge to 7-2.

"We played a very poor 40 minutes," said Denis Potvin, whose power play goal at 19:17 of the third period brought the Islanders within one, after short-handed scores by Billy Carroll and Anders Kallur and another goal by Bossy.

Bossy might have had three goals and taken his club into overtime. With seven seconds left, he seemed to poke the puck beneath goalie Ed Mio's pads, which would have tied the score. But referee Bruce Hood said he had lost sight of the puck and had whistled play dead.

Mio claimed he had frozen the puck and Bossy shuffled it loose. And Arbour said he had no view at all of what had happened, and didn't dwell on it. "It would've been a great comeback, but the referee blew the whistle," he said.

Rangers Coach Herb Brooks was openly relieved the comeback had fallen short.

"It was obviously the most important objective, to get a win here," he said. "It's tainted a little by the third period when we let the whole thing get away from us. Maybe it's good for us in the sense that we know we've got to play a 60-minute ball game against a team like the Islanders."

Brooks said his team left the ice with a sense of fear after the third period. He joked about his own fear when the Islanders threatened to tie the game. "I was thinking, did I renew all my insurance licenses? I'm a hell of an insurance salesman, and I figured, what am I doing coaching hockey?" he said.

But for 40 minutes, Brooks' coaching system effectively contained the Islanders. Mio's goaltending was sharp, and the forecheckers did the job they had ignored for the previous two games.

"We had some good chances in the third period, but their goalie also had some good saves," said Duguay. "We came into this game after a really good practice Saturday, when we realized mentally we hadn't been prepared for this series, the way we were against the (Philadelphia) Flyers. Hopefully, we'll learn from the mistakes of the third period tonight and play even better tomorrow."