I just replaced my belt tensioner assy.
Before that a while back I replaced the vibration damper, and the belt squeeled under hard acceleration and in high rpm's. The belt was a gatorback and relatively new.
I always assumed it was a light rust on the belt area of the vd pully that caused the slip, but after putting the new tensioner assy on I realized how weak it was.
Just another thing to consider.
Edit: you need a straight edge to check alignment