i had this issue also, turns out the crankshaft pulley wasnt pushed in all the way so it was slightly out of align. It might look like its aligned by eye but it was off by a hair so at high rpm's it would come off. i lost so many ps belts i was going to get rid of the ps all together. i just used an impact gun on the crank pulley(even though thats frowned upon) to tighten it down more.
Also maybe the pulley the belt rides on is bent.