I'll through out another check,
ALL the pulleys should be "V" shaped not "U" shaped from wear, including and especially, the drive clutch.
As the pulleys wear, they tend to cause the sharp "V" of the belt bottom to loose their edges and mimic the "U" shape of the worm pulleys.
Well, pretty quick the belt rolls over in the pulley, because it begins to look like a rope and destroys itself.
BTDT saw it many times.
The 1/4 twists the belts makes, causes the wear, its just the nature of that style of drive.
