Mower deck 2003 Cub Cadet tank M60 KW keeps throwing the pto belt...

I showed a video of my issue to my mechanic, he told me over the years of wear and tear, the deck shifts backwards and loosens the tension of the belt. A quick solution is to use a tighter belt, maybe 1 to 2 inches. The long-term solution is to weld the deck where the bolts created an oval hole instead of a circular hole. He stated that the new tanks allow the owner to adjust the mower deck to tighten the belt.

check out z-force service manual 2003-2006 idler arm hardware for being loose page 3, i would check all hardware on deck for being loose especially idler pulleys, etc.