I'm working on a Scag 61" Tiger Cub w/Advantage Deck. The deck belt keeps coming off each time the PTO is engaged. I tried engaging the PTO at all different RPMs and I've checked all spindles and pulleys for any wobble, sticking, etc. Belt is also fine. Everything is fine - belt rotates fine when I put it on and rotate it by hand - but the second the PTO engages - the whole thing comes off. I believe I have it on right - checked it against the diagram that's written on the deck and the tension on the belt is good too. This one has me stumped. Please help me.