Yvon Lambert converted Mario Tremblay's pass at 9:33 of overtime tonight and the Montreal Canadiens clung to the Stanley Cup for at least another week.

The Boston Bruins, who never trailed until the 5-4 game was history, lost their 15th straight game in the Forum and can spend the summer reciting "ifs" as they recall this great seventh game of a grueling semifinal series.

In their nightmares, they will see an extra played on the ice, for it was a power play created by a too-many-men penalty that enabled Guy Lafleur to tie it for the Canadiens with only 74 seconds left in regulation time.

The Canadiens had little time to celebrate and their joy was tempered by losing defenseman Guy Lapointe. He tore ligaments in his left knee in the third period and will miss the final series opening here Saturday night against the New York Rangers.

"It was a super game, spectacular," Boston Coach Don Cherry said. "When we were ahead 3-1, we had a lot of chances to make it 4-1, and we had a lot of chances in overtime, too. I'm not going to talk about the referee and overshadow our good effort, but when it was 3-1 you could tell he was just waiting to call one against us."

Two goals by Wayne Cashman had built that 3-1 lead and the way Gille Gilbert - the game's No. 1 star - was performing in the Boston net it appeared that Montreal's three-year string of Stanley Cup championships might be snapping.

Montreal goalie Ken Dryden made good stops on Terry O'Reilly and Rick Middleton to keep the margin from escalating and then Lafleur helped to find some vulnerable spots in Gilbert's armor. Lafleur skated past defenseman Al Sims and around the net, then passed to Mark Napier, who beat Gilbert with a drive from the right-wing circle.

The occurred at 6:10 of the third period and 61 seconds later referee Bob Myers whistled his first penalty in 17 minutes, nailing Boston's Dick Redmond for hooking Jacques Lemaire from behind.

Lapointe's 50-footer, off another Lafleur pass from behind the net, produced the tying goal and it appeared that the Canadiens had the momentum to put the Bruins away.

Lapointe was injured, however, in a scuffle with Middleton and Mike Milbury near the boards. After he was wheeled off on a stretcher, the Canadiens suffered an obvious letdown. Middleton, after avoiding a check by Serge Savard behind the Montreal net, was able to circle in front and carom a shot off Dryden's right arm.

Only 3:59 remained and the way Savard flubbed two good scoring chances, it was obvious the Canadiens were nervous. But Boston inexplicable messed up a line change and gave Montreal a power-play opportunity, the kind Cherry always complains about, because the Canadiens take such potent advantage.

Nobody was pinpointing blame, but it appeared left wing Middleton came on the ice and his opposite number, Stan Jonathan, did not leave.

"It was my fault, a man just jumped on," Cherry said. "He must have heard me or something like that. Too many men on the ice is the coach's fault. I grabbed two more or we would have had eight guys on the ice."

The penalty was noted with 2:34 left, and 1:14 was showing on the clock when Lafleur took a drop pass from Lemaire and beat Gilbert with a 40-footer into the far corner from the outer edge of the right-wing circle.

Montreal had 11 shots in the overtime, the Bruins three. But the Canadiens' reign could have ended after just two minutes if Dryden had not made a superb stop on Don Marcotte, set up in close by Peter McNab.

When Middleton tried to carry the puck around Savard, the defenseman foiled the breakaway try and started the winning play. Tremblay carried down the right wing and, skating along the rear boards, sent a perfect setup to Lambert just outside the crease.

Suddenly, Boston was dead, and as the Canadiens happily jumped on each other, the crowd of 17,453 was raising the decibel level to a dangerous area.

This was the first time since 1971 that Montreal had been involved in a seventh game and it was the first in the Forum since 1965. The NHL went to a seven-game format in 1939 and this was only the eight time a seventh game had gone into overtime, resulting in almost unbearable tension.

If anyone is maintaining a list of the 10 most exciting hockey games ever played, it may be due for some revision. CAPTION: Picture, Wayne Cashman of the Bruins watches a teammate Rick Middleton's shot eludes diving Canadien goaltender Ken Dryden. UPI