Jake Cassidy and Jack Lambert netted in the opening 22 minutes as the Quakers made it four straight wins at Blackwell Meadows in the National League North, pulling themselves well and truly into the play-off race.
But for Boston it was a case of the same old problem coming back to haunt them.
Too many times this season they have found themselves chasing the game after shipping early on.
"I thought we started slowly and looked a yard off the pace," said manager Paul Cox.
"Especially in the early stages of the game.
"Conceded once again a really, really sloppy goal. The first one just put us on the back foot and once again you're chasing the game.
"We just looked a little bit off it today. We never really got our game going."
Danny Elliott netted his 24th of the season - his third in as many games - from the spot to give United hope with half an hour to go, after winning the spotkick himself.
But a 78th-minute red-card for captain Luke Shiels for a foul on Cassidy handed the hosts yet another numerical advantage.
"We had a right good go second half and even when we went down to 10 men it looked as though the team were going to go and try to get a second," added Cox, who felt Shiels deserved a booking but not a straight red.
"I think the skipper's gone in wholeheartedly. It was there to be won and both of them were in the air.
"You see it time and time again."
Shiels and Jake Wright snr will both be suspended for Saturday's arrival of Blyth Spartans, with fellow central defender Scott Garner injured.
With transfer deadline passed, some tinkering will be needed in training this week.
Boston dropped to ninth, but just two points off the play-off spots, while the Quakers are a place and a point behind them.
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