This may sound silly to some, but even something as simple as the brand of belt may be to blame - not all belts are created equally.

I've found that the DAYCO belts on my '62 Corvair are more likely to get thrown off the pulleys then any other brand (they're actually a tiny bit wider & don't sit in the groove as well).

The Dayco's would also shread on my last car, a Dodge Stealth, where all pulleys checked fine. Could easily be a similar issue here.

Dayco's are usually in a black/grey package
Kelley-Springfield in a green/white package
not sure about the other brands.
Hope it helps.
~Brenda