The last time I had a problem with a belt popping off, it was a bent crank pully.
It would run fine and at all speeds as long as I didn't race the engine to get to speed.
As soon as I floored it though, the belt would pop off leaving the water pump as a paperweight..useless.
If the belt is reasonably new and of good quality AND the alignment of the pullys is good, I would watch during idle and see if any pulleys are warped or bent (might even need to use a runout gauge), any brackets loose enough to move under pressure of fast revving or that all the spacers for all the accessories are in place, no nicks in the pullys etc that catch the belt and ride it out of the groove.
After all that checks out good and it still does it, I would get the best quality belts I can find and use them.