John L.
Charlie is right, belt issues are almost always mule drive related.
Leveling your deck fore and aft usually HELPS FIX belt issues, as it better aligns the center pulley with the mule drive.
Look for sticks jammed into the "v" of the pulleys, misaligned mule drive idlers, too loose of mule drive idlers (tighten them up some more), slipping PTO clutch over heating belt, severe rust on pulleys, oil/grease on belt, etc.
Be VERY CRITICAL of alignment, and don;t be afraid to "tweak" the mule drive some with a 12" adjustable wrench to make alignment adjustments to the idler bracket.
Also, try removing the spring on the return side idler and replacing with eye bolt......sometimes the springs get weak and cause "flop" in the belt, especially on start-up.
Have someone else engage the deck at various RPMs and watch how the belt behaves from several angles....you'll get it sorted out!!!!