Part of the Islanders’ identity the past two seasons was that of a team that you didn’t want to let hang around, lest it outwork you late for a point or two.

This season, it seems a tie game heading to the third period is the Islanders opponents’ best friend.

For a second straight game and fifth time this season, the Islanders were even entering the third period and came away with nothing last night at Barclays Center. Matt Niskanen’s goal 1:34 into third snapped a tie and the Capitals took a 4-2 win from the Islanders, who had many of the same culprits among their forwards missing in action.

“Obviously it’s frustrating to keep losing games the same way,” said Jaroslav Halak, who could do nothing on Niskanen’s blast off the crossbar and in but let Niskanen’s point shot squeeze through him with 1:58 to go to seal it. “Every game it seems like we’re in it after the second, we go into the third and we find a way to lose it. That’s what’s going on right now.”

With the Sabres and Hurricanes winning on Tuesday, the Isles loss dumped them back into the Eastern Conference cellar, a dozen points behind the Caps. Even when it was 2-2 after the second thanks to Nikolay Kulemin’s deflection of Nick Leddy’s point shot in the second and Brock Nelson’s wrister in the first, the Isles were swimming upstream, outshot 44-31 through 40 minutes, 24-16 in shots on net.

Jack Capuano has limited options among his forwards, but he hinted that some of his struggling players may need to take a seat. Andrew Ladd and Jason Chimera combined for zero shots and both were on the ice and out of position on Niskanen’s game-winner, with Ladd shading to the same side Chimera was on, allowing Niskanen to walk in and blast away.

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“It’s frustrating. You know the talent’s there, but you don’t know where it’s gone,” Capuano said, declining to name names. “I’m losing my patience a little bit here . . . It may be best to have them sit back and watch a game. We’ve done that in the past.”

Some Islanders raised their games after Niskanen’s first goal, particularly the John Tavares-Anders Lee-Josh Bailey line. Bailey had two chances from in close that Braden Holtby turned aside, then the Caps goaltender denied Tavares on a rebound in the slot midway through the third.

But the damage was done. This wasn’t a complete third-period meltdown as in Columbus on Saturday, when a 2-2 game turned into a 6-2 shelling. But the Islanders are in no position to let down at any point in any game, not with 15 teams above them in the East.

“We’re in a situation where we can’t afford to lose a couple in a row,” Dennis Seidenberg said. “We played a decent game and a couple breakdowns cost us.”

If there is still time to save this Islanders season that’s a little over one-third gone, the third periods have to be better. The Isles’ identity rests on being that sort of team.

“When we have a little streak like the (5-0-1 run recently), we have to be able to bounce back quickly from a couple like this,” Lee said. “We’ve got to get out of this.”