Shredded belt...
 

Took time to rebrush the alternator in my father's 99 QC this weekend. Found a shredded belt, with only about four of the ribs remaining. R&R'd the alternator, and installed a new belt. Bumped the starter a few times and the belt immediately tried to jump off the AC pulley, indicating the idler/idler pulley was off alignment.

Pulled the idler figuring it was toast with just shy of 300k miles, inspected it thoroughly...nothing obviously wrong. However, there was a batch of corrosion on the idler where it mated to the block. Took some time to chip all the corrosion off, it was galvanic corrosion between the iron block and aluminum idler arm. It was only on one part of the idler arm, and about 1/16" thick...more than enough to throw the alignment off.

Pulled the idler pulley, and greased the bearings just because.

Put everything back together, and it seems fine...its seems to be tracking on the pulleys fine now.

So just an FYI, if you have any miles on and have a belt alignment issue, don't automatically assume the tensioner or tensioner pulley are faulty.

ive seen power steering pullys where the belt will be fine and a couiple days later make noise and the pully backed off the pump a little