The New York Islanders have a bad habit of losing concentration when they build a lead. It happened again tonight, but it was not costly. The Islanders were too far ahead to be bothered.

Converting all five of their power-play chances and adding a sixth score while short-handed, the Islanders thrashed the Flyers, 6-2, and grabbed a 2-1 lead in the best-of-seven, Stanley Cup final. Game four is scheduled here Monday night.

Ahead by 6-0 after 49 minutes, the Islanders permitted scores by Bob Clarke and Mike Busniuk and there was a stir of anxiety among the 14,995 faithful when Tom Gorence broke in alone with 6 1/2 minutes left. Gorence shot high, however, and everyone relaxed.

"It seems as if we can't stand prosperity," said Islander defenseman Gord Lane."We get up on teams and then we relax and let them close up. But this time we had a pretty good edge."

The shots on goal read 33-17 after the first two periods as the Islanders made amends for that 8-3 shellacking they received in Philadelphia Thursday night. The Flyers had the upper hand this time for exactly one minute, then they went downhill fast.

Helping their collapse were a couple of injuries, as winger Paul Holmgren (knee) and defenseman Jim Watson (shoulder) departed in the first period. Penalties to defensemen caused added grief in the second period and winger Tom Gorence played horribly after taking an unpenalized five-stitch slash in the neck from Islander Bob Nystrom.

In the first minute, Flyer Ken Linseman sailed past Denis Potvin and carried the puck into the crease, where he was stopped by goalie Bill Smith. Then a weird carom presented the Flyers a two-on-nobody break, but Smith dove out to knock the puck away from Holmgren, who was hurt in the ensuing pileup.

Brian Sutter was chased at the 61-second mark for hooking and the Flyers confidently awaited the power play. It backfired when Al Hill lost the puck to Bob Bourne and Bourne fed Lorne Henning on a two-on-one break. Henning's drive struck goalie Phil Myre's pad and skidded into the corner of the net for the Islander's seventh short-handed playoff goal, a Stanley Cup record.

It was the first shot at Myre, obviously rusty in his first appearance of the final after posting a 5-0 record in earlier contests. He did not get better and the shift from Pete Peeters opened Coach Pat Quinn to criticism, on a day when he was selected NHL coach of the year.

Potvin, a two-goal scorer for the second time in the series, connected with Flyer Bob Kelly off for tripping to make it 2-0, then Quinn appeared to make another tactical mistake.

Referee Bob Myers chased Philadelphia's Rick McLeish and Bob Dailey 32 seconds apart to set up the first two-man advantage of the series. Quinn sent out two penalty-killing forwards, Clarke and Bill Barber, who played up front, leaving only defenseman Behn Wilson to protect Myre.

Mike Bossy faked Barber and sent a perfect pass to Bryan Trottier, unguarded in front of Myre. Trottier waited until Myre went down, then backhanded the puck over him: 3-0. Before Dailey left the box, Bossy ripped a 30-footer past the right shoulder of the beleaguered Myre, who came out with little hope of cutting the angle, and it was 4-0.

New York dominated the second period, firing 21 shots at Myre while Smith was tested only eight times. Myre played valiantly, but he was beaten twice, by Clark Gillies and Potvin. Both goals came with Flyers in the box, as the Islanders' five power-play scores came within one of the Stanley Cup record for one game.

Each team served 42 minutes in penalties during the second period, which saw four majors and four misconducts.

First Nystrom gave Flyer Mike Busniuk a pretty good beating, then Lane and Jack McIllhargey tangled in a heavyweight main event. Myers became weary of the belligerence, particularly the refusal of secondary combatants to loosen hold of jerseys. So he assessed Iane, McIllhargey, Gorence and Islander Garry Howatt 10-minute misconducts to restore control.

Combined with the realization that the outcome was certain, this remedy served to cool off most of the players. The third period proceeded in quiet fashion, until the last 30 seconds.

Then Wilson and Sutter resumed a feud building since the start of the series. Both were pointed toward the exits by the weary linesmen and Sutter earned the final pointless point of the long, long night by extending his index finger in the direction of the scoreboard.