Alexander Semin had a goal and an assist as the 3.
Brooks Laich, Matt Hendricks and Jay Beagle all scored for the Capitals, who have won their past two and with 74 points sit in the eighth spot in the Eastern Conference and are just one point behind Florida for the top spot in the Southeast Division.
Tomas Vokoun turned aside 30 shots for the win.
Brad Marchand had a goal and an assist and Johnny Boychuk and Milan Lucic each lit the lamp for the Bruins, who had a two-game winning streak stopped, but will hold on to the top position in the Northeast Division and second overall in the conference.
Tim Thomas was tagged with all four goals on 30 shots in the loss.