12.7 Detroit belt issue
We run these motors and have had the same problem. After changing a bunch or parts and going through numerous belts, we finally figured out that if you use an 8 groove pully you need to use a 6 groove belt. This allows it to have a little wiggle room and does not walk over the edge of the pulley. Sounds crazy, I know, but is has worked so far. The current belts are coming up on a year and still in good condition.