Serpentine Belt slipping off
 

I have a '99 TJ Sport (I6). It has an after-market A/C system installed. The serpentine belt has started to slip off. The first time was mid summer. Then it happened once again in September. After that last time, I checked it every couple of weeks or so. Once it looked like it was getting ready to slip of the water pump pulley. On my Jeep, this is a smooth pulley and the back (non-toothed side) of the belt rides along this. I readjusted the belt so it was riding more to the center of the pulley. Today it happened again. This time the belt got a little chewed up. I was able to put it back on and make it to the parts store. I got a new belt, $42 -ouch, and a new adjuster pulley. I put the new belt and pulley on and it looks like the belt is moving slightly toward the edge of the pulley again-towards the front of the Jeep. Is there an adjustment I can make to move the water pump pulley away from the block/toward the radiator? I'm thinking that somehow the pulleys aren't all in the same plane.

Help, please.