Take off the squeaking belt and turn each pulley by hand to see which one is binding up. Fix that one.

Have you tried a bit of belt dressing?

Perhaps the belt has been contaminated by one of the vehicle's other fluids.

is the water pump leaking coolant on belt

Have you checked the tensioner?

It only happens when the car sits for awhile, mostly overnight when I start it up in the morning. I replaced the power steering pump, replaced the belt, and tightened it recently. I can "feel" the belt slipping when I turn the wheel.