Last night, for the third time this season, the Washington Capitals yielded five goals in a period. A team rarely can overcome that kind of defensive collapse, and the Capitals were no exception, dropping a 7-5 decision to the New York Islanders before 18,130 at Capital Centre.

The Islanders' Bryan Trottier scored three goals for his 15th career hat trick, and Mike Bossy had four points to reach 1,000 over 8 1/2 NHL seasons.

Bossy's 508th goal, which began the Islanders' comeback from an early 2-0 deficit, moved him past Jean Beliveau into 10th place on the all-time NHL list. It was appropriate for Bossy to reach these milestones at Capital Centre; he was the All-Star Game MVP here in 1982 and has scored 50 goals in 48 regular-season games against Washington.

"I've had quite a large amount of success here," Bossy said. "It's fun, of course, to put these numbers up on the board. Once my career's over and I look back and see I was a better goal-scorer than some great hockey players, I'm sure it will mean more then.

"The big thing is that games like this are confidence builders. We served notice to ourselves that we have to work harder and concentrate more to play better as a team. We haven't been consistent as a team, but except for Montreal (a 3-0 loss), we've been doing pretty well lately."

The Capitals still are nine points ahead of New York with a game in hand, so they are not panicking, but they were disappointed to lose like this in front of a sellout crowd at home.

Coach Bryan Murray replaced starting goaltender Pete Peeters with Al Jensen at the start of the third period, after Peeters had yielded six goals on 18 shots. But Murray said he was just trying to shake things up, not put the onus on Peeters for New York's five-goal second period.

"Pete wasn't great, but he was made to look a lot worse than he played," Murray said. "They scored on a lot of second shots and tip-ins. Every goal except the sixth came when one of their men was aggressive going to the net. They were going through our people to get a deflection or a rebound."

The Islanders were fussing and fuming and probably were vulnerable, too, after Scott Stevens lifted Washington into a 4-4 tie with 5:44 left in the second period.

Stevens' shot from the left-wing boards squirted off the pads of New York goalie Bill Smith, who was distracted by a goalmouth battle between teammate Paul Boutilier and Washington's Greg Adams. Smith protested in vain to referee Ron Fournier that Adams had intruded on his crease before the puck slipped through.

Moments later, Larry Murphy rattled a post from the right point and the puck remained loose in front until Fournier whistled coincidental minors to Smith and Mike Gartner for slashing.

Not long after, Boutilier's drive from the left-wing boards was deflected by Brent Sutter, and the Islanders were ahead to stay. Before the period ended, it became 6-4 as Mikko Makela lined a 50-footer past Peeters' glove.

Jensen, as Murray hoped, gave Washington a lift, foiling John Tonelli's breakaway for his first save. But Smith, beating the Capitals for a league-high 21st time, was a spoiler at the other end with big saves on Grant Martin and Gartner.

Jensen was lifted for a sixth skater with 1:43 remaining, and for the first time since Oct. 16, the tactic worked for Washington. Gartner scored his second goal of the game at the finish of some sharp passing by Bob Carpenter and Dave Christian.

With 65 seconds left, the Capitals began to hope for a miracle finish, only to be deflated a mere six seconds later when Trottier completed his hat trick into the empty net, with Bossy assisting for point No. 1,000.

"Boss and Trots together are getting that chemistry going again," said Coach Al Arbour.

Murray's viewpoint was slightly different: "Trottier and Bossy always seem to play well against us, but we let them play well. On the one play, we're 10 feet away from Bossy and backing away from him. You can't stop him like that."

Gartner and Christian, who lead the Capitals with 26 goals each, provided the 2-0 lead in the first period. Bossy halved it before the intermission, and Trottier created a 2-2 tie after 27 seconds of the second period.

Craig Laughlin's 15th goal gave Washington a brief 3-2 lead, but it was the first of three goals in 24 seconds, the fastest three in Capitals history, and the Islanders got the next two.

Laughlin deflected Murphy's drive at 5:45. Trottier knocked a twice-redirected puck past Peeters at 5:58 as Alan Haworth pulled it out of the net an instant too late. Then Greg Gilbert put New York in front, 4-3, with a low, screened shot at 6:09.

Although the fans jeered when Peeters made a routine save, the goalie took his departure philosophically.

"I seemed to be on top of the puck and on top of the play," Peeters said. "But they seemed to get their sticks in the way, and they sure got the bounces. Those things are going to happen. Thank goodness it didn't happen in the playoffs."

Left wing Jorgen Pettersson was flattened by the Islanders' Denis Potvin in the second period and left with a bruise on the inside of his left knee. Pettersson is expected to miss tonight's game at Minnesota, and Mark Taylor was called up from Binghamton as an emergency replacement.

"I think it was his (Potvin's knee) that hit me," Pettersson said. "Bengt (Gustafsson) left the puck for me, I came across and he (Potvin) came on the side and I got caught on him.

"I felt okay the next time out, but when I made a fast turn up ice, I felt something and I figured I'd better get off. Hopefully, it will just be a day or two."

Washington captain Rod Langway was struck in the mouth by a puck shot five feet away by Pat LaFontaine. Treated at the bench, Langway did not miss a shift. Black Hawks 5, Sabres 3

Marc Bergevin's third-period goal led Chicago to the victory in Buffalo.

The loss was the first for Sabres Coach Scotty Bowman, who replaced Jim Schoenfeld last Friday. The Sabres had won three straight under Bowman.

For the Black Hawks, it was the first victory in Buffalo since Feb. 22, 1979.

Bergevin scored on a two-on-one break, beating Buffalo goalie Tom Barrasso and giving the Black Hawks a 4-3 lead. Denis Savard scored an insurance goal for Chicago with less than two minutes left in the game. Oilers 7, Devils 6

Wayne Gretzky stole the puck from Craig Wolanin and beat Craig Billington from close range at 1:29 of overtime to lift Edmonton over visiting New Jersey.

Mel Bridgman gave the Devils a 5-3 lead 3:49 into the third period, beating Oilers goalie Grant Fuhr on a breakaway. Mark Messier cut Edmonton's deficit to one goal five minutes into the period.

Kirk Muller restored the Devils' two-goal lead 6:04 into the period, but Messier scored again 3:54 later to pull the Oilers to 6-5, and Mike Krushelnyski tied it at 6-6 to send the game into overtime.

Trailing, 3-0, after 20 minutes, the Devils scored four straight second-period goals to take a 4-3 lead.

Canucks 4, Penguins 3: Goaltender Richard Brodeur made several spectacular saves in the third period in Vancouver to help beat Pittsburgh.

Brodeur stopped eight shots in the scoreless final period as the Canucks protected a one-goal lead. He made two brilliant saves on Doug Shedden and another on Dave Hannan.

The Canucks came from behind to take the lead late in the second period on a power-play goal by defenseman Rick Lanz and the winner by Thomas Gradin, who deflected a shot by Lanz.

The victory allowed Vancouver to move into a third-place tie with Los Angeles in the Smythe Division standings, a point ahead of Winnipeg.

Mario Lemieux led the Penguins with a goal and two assists to increase his total to 82 points in 47 games.

Brodeur finished with 25 saves. Pittsburgh's Roberto Romano had 29