Volvo Repair: serpentine belt doesnt stay on 94 850 non-turbo, serpentine belt, pulleys

Question
The serpentine belt came off while driving the other day. I had to cut it off as it was jammed behind the tensioner.  While off I checked the rotation of the pulleys on the alternator, water pump and powr steering pump.  THe PS pulley wobbled slightly and only spin a couple of times before stopping. The other pulleys rotated freely and spun numerous times before stopping.  I installed a new belt and upon start up the belt popped loudly and again jammed behind the tensioner.The car has 130000 miles and had the belt tensioner replaced at 70000 miles.  Should I change the PS pump?

Answer
Greg, they dont usually go bad but if you have that kind of wobble in it, it definately will do that with the belt. Well the water pump is driven by the timing belt so you must be talking about the idler pulley.   If everything else is free then that is the culprit.  Roger