So much for the good vibes.

After scoring three third-period goals in St. Louis for their first comeback win of the season on Tuesday, the Bruins allowed three unanswered goals in the final 5:45 of the second period and went down 7-2 to the Dallas Stars at the American Airlines Center on Thursday.

The Stars, who beat the B’s at the Garden in October, outclassed the B’s for the second time to sweep the season series.

Boston’s power play, ranked dead last in the league, continues to crater. It missed on a pair of chances, both of which came when it was still a game. They’ve cashed in on just four of their last 48 chances.

The B’s have not gotten nearly enough positive impact on their two big signings. The play of Nikita Zadorov, who leads the league in minor penalties, has been mercurial. Center Elias Lindholm has not scored a goal in over a month, dating back to the third game of the season.

And Jeremy Swayman, who missed all of training camp as his protracted contract negotiations dragged on, is not disguising any warts. The seven goals allowed were a career high for him.

Frankly, it’s amazing they’re the NHL’s version of .500 at 8-8-2. They’re back at it on Saturday against the Blues again. If there are answers to what’s happening with this team, coach Jim Montgomery hasn’t found any yet.

“We’ll review the tape, we’ll have a meeting and we’ll get ready for St. Louis on Saturday,”  Montgomery told NESN. “Sometimes it takes a while for us to get our legs and get going in the right direction consistently. It’s been an issue so far.”

It was an eventful first period in which the B’s actually played pretty well, though they were victimized by some bad luck early.

The bad luck started when the Stars got on the board just 1:09 in. Charlie McAvoy was going back stride for stride with Mason Marchment before he blew a tire. He got up and chased Marchment behind the Bruin net but never completely caught up to the Star to shut the play down. On the left side, Marchment shoveled the puck to Matt Duchene, who beat Swayman with a one-timer to the shortside.

The Stars took a 2-0 lead at 6:36 on a penalty shot that was debatable. In the offensive zone, Mason Lohrei was rushed into a shot by Evgenii Dadonov and he just fired the puck wide. Dadonov blew the zone and took a pass for a breakaway. It appeared as though Lohrei had legally hectored Dadonov from behind enough to thwart a good scoring chance, but the refs did not see it that way. Dadonov was awarded a penalty shot, and he beat Swayman under the blocker arm.

Then the B’s, already without Hampus Lindholm and Andrew Peeke, lost Brandon Carlo when Jamie Benn drilled him from behind into the end boards. Carlo, who has a well-documented history of concussions, was shaken up and would eventually leave the game for the remainder of the period, though he returned at the start of the second. Zadorov would not let it slide and gave Benn a pretty good pummeling.

Benn was initially given a five-minute major, which seemed reasonable, but the refs decided to reduce it to two minutes after a video review.

The B’s, their 32nd ranked power play in a season-long funk, could not make Dallas pay on the PP. But they did cut the lead in half before the period was out. Trent Frederic, who was pointless in the previous eight games, did a good job of controlling the puck and fed Charlie Coyle, who bet Jake Oettiinger with a quick one-timer at 17:48. It was Coyle’s third of the year and first 5-on-5 tally.

The B’s flurried at the end of the period, but they could not get the equalizer.

David Pastrnak was tripped early in the second period but, again, the B’s could do nothing with the advantage. As it happened, that was their last best chance to make it a game.

Through the second period, the Stars began to establish some offensive zone time and, eventually, the B’s committed some self-inflicted mistakes that led to a pair of Dallas goals in 1:41.

First, John Beecher iced the puck from the neutral zone and the Stars were able to get some of their big guns on the ice. At 14:14, Logan Stankoven cleaned up a loose puck off a Benn shot for a 3-1 lead.

Then, Zadorov’s soft breakout pass up the middle was picked off by Dallas for a quick reload. Colin Blackwell’s pass deflected off Zadorov and Oskar Back just flicked his stick at the puck and popped it over Swayman’s glove arm.

The disintegration was complete with 11 seconds left in the period. The B’s look like they were ready to head into the locker room to regroup but the Stars were not done. Dadonov was able to walk down into the left circle and beat Swayman for another shortside goal.

Just 47 seconds into the third, the B’s missed two chances to get the puck out and then Roope Hintz outmuscled both Carlo and Lohrei in front to sneak a shot between Swayman’s pads.

Pastrnak answered with one quickly after that, but it wasn’t going to change the outcome in this one. Marchment added another one for final blowout score.