4.6is Throwing A/C Belt

[XR4Tim]

The air conditioning belt on my 4.6 keeps skipping forward a rib (or two) on the crank pulley. This causes it to rub against the other belt, one time pushing it into the fan and destroying it (making for a fun night on the side of the highway). I replaced the tensioner, thinking that perhaps the roller had tapered, and was working the belt forward, but a few days later it had moved forward on the crank pulley again. So I'm guessing it's got to be a problem with the A/C compressor or the crank pulley being out of alignment. The odd thing is that the first 4 months that I had the car, there was no issue, so something's changed recently.
Has anybody had this issue before?