I would also check the transmission bolts and make sure there tight.Is it possible that when you replaced the new belt that your alignment was off just enough that over time, you've worn the edge of the belt off? I would remove the belt and expose the drive pully to see if there is any slack in it, i.e. worn bearings. If it's snug and there's no obvious movement, replace and re-align the belt so that it travels in the center of both the drive and rear pully. Just food for thought.
Have you ever thought that maybe it was never aligned properly to begin with? A lot would have to go wrong for the nut to come loose on the drive pully. Once the nut it tightened, there should be 2 small screws that are put in place beside the 2 flat sides of the large nut, keeping the nut from turning and coming loose.It has never set in the middle of the front pulley. it was about 1/8" from the edge
