P.A. Parenteau, left, slips the game-winner pas Capitals goalie Tomas Vokoun. (Kathy Kmonicek/Associated Press)

The Washington Capitals came to Nassau Veterans Memorial Coliseum aware that the New York Islanders were mired in a six-game losing streak and eager to climb out of their slump.

Despite that knowledge, and despite jumping out to a two-goal lead in the first period, the Capitals succumbed to New York’s push. In fact, they helped it along by deviating from their game plan, by coughing up the puck and by getting out-hustled in the defensive zone.

P.A. Parenteau scored the game-winner with 1 minute 46 seconds remaining in the Islanders’ 5-3 victory on a play that exemplified the Capitals’ problems. A point shot by Milan Jurcina squeaked through Tomas Vokoun’s pads and none of the skaters picked up Parenteau lurking around the crease, where he poked the puck home.

“I think we have a pretty good start, score two goals,” said Alex Ovechkin, who recorded a goal and three shots. “After that, we just stopped playing and give them opportunity to score goals, and they’re young, fresh and they want win. After first period, we just stopped playing.”

Rick DiPietro made 25 saves for the Islanders while Vokoun finished with 30 in an uneven outing. After receiving Friday night off against the Carolina Hurricanes, Vokoun made some important stops early on against the Islanders, but three of the four goals he allowed were soft.

The Capitals initially appeared to pick up Saturday where they left off from Friday’s win against the Hurricanes. Joel Ward made it 1-0 only 5:24 into the game on a heads-up play in which he batted a rebound out of midair, turned and flung it back toward the net, catching DiPietro by surprise. Ovechkin upped the advantage to 2-0 with a rocket of a shot on a setup by Nicklas Backstrom with less than two minutes remaining in the first.

“We had them,” Coach Bruce Boudreau said. “I think if we had of come out in the second period as we had come out in the first, and I think we knew that if we could have got that next goal it probably would have been over, but we didn’t, and once we let them get back in the game. . . . You let a team in, especially a team that’s been losing, and they feel it and you’re in trouble.”

Only 1:41 into the second, Frans Nielsen put the Islanders on the scoreboard at 2-1 when he retrieved the rebound of his own shot and beat Vokoun through the five-hole. The Capitals pressed in search of a goal but instead started turning over the puck and giving New York opportunities. With just less than five minutes remaining in the second, a seemingly harmless shot by Brian Rolston from the point beat Vokoun five-hole again and tied the game at 2.

“The first two and the fourth one, you can’t win when your goalie gives up three bad goals,” Vokoun said. “As much as we play hard, tough on back-to-back nights, I didn’t help the guys much, obviously.”

Matt Martin gave the Islanders their first lead of the night 6:28 into the third when he outworked Dennis Wideman, who finished a team-worst minus-four on the night, in the crease to take control of a rebound.

While a power-play goal by Brooks Laich made it 3-3 near the midway point of the third, it couldn’t stem the defensive lapses that continued to give chances to New York. Parenteau sneaked behind the defensive pairing of Wideman and Roman Hamrlik to find the puck loose behind Vokoun for the eventual game-winner. John Tavares made it 5-3 with an empty-net goal.

“Second period we started to beat ourselves, throwing pucks through the middle, not getting it deep and that’s what happens — they get the puck on transition and score some goals,” Karl Alzner said. “It’s the same thing that happened the Edmonton game, the Vancouver game same thing, and this one. It’s a little bit of a trend where we start to do these things to ourselves and teams are capitalizing on it.”

Capitals notes: Mike Green missed a fifth consecutive game with a twisted right ankle. . . . Rookie Cody Eakin and D.J. King were healthy scratches.