If pulleys are not fully aligned this may flip or throw a belt off. If it is just flipping in the pulley it may be a wrong section belt for the pulley eg an A section belt would be too slim for a B section pulley and would cause belt to not grip properly and possible flip in the pulley. Try going on John Deere website most parts manuals etc are downloadable http://jdpc.deere.com or just try google search for John Deere parts and a link should come up. Hope this helps
Smudge
February 2014