I had this same problem. Had the belt on my car for years shifting at 6700rpms, hitting the rev limiter occasionally with no problems. Changing oil one day and noticed the bottom was missing some chunks. I put and new belt on of the same part number. First time out shifting it came off. I checked pulleys, mounts and tried a few different brands of belts, nothing worked, if I hit the rev limiter it would come off. Never did figure it out, I changed over to serpentine belt.