When I replaced the little belt for the wp I saw it could be put on out of line. That is, with the ridges on the belt lined up with the wrong grooves. The belt would end up a little out of line from one pulley to another. Could be the same for the big one, I don't remember.
With that grroved belt, it can not move from side to side while running. Good luck- I would definitely get at it right away.
