Bob Weir -- Grateful Dead Guitarist Collapses on Stage

Bob Weir -- founding member of the Grateful Dead who now plays guitar for spinoff band Furthur -- had to be helped offstage last night in Port Chester, NY, after collapsing during a song after suffering some serious pain in his shoulder.

The band, which also features former Dead bassist Phil Lesh, was playing the Dead song "Unbroken Chain" when Weir wobbled for a few moments before going down (00:45 mark in the video).

Weir was immediately helped up by crew members and given a chair to sit in on stage. The band finished the song with Weir ... but helped him off the stage as soon as the jam ended.

Lesh returned to the stage moments later and said Weir had been bothered by a strained shoulder. They finished the set without him.