Murray riding mower problem

Have a Murray 20 HP riding mower, model 42572x8A (purchased new in 2001). After about 10 minutes of riding it just slows down to a crawl whether the blades are engaged or not. Went through the following items but am stumped. Any ideas??? Looks like either a motion belt may be stretching in the heat or possible transaxle problems? The blades and engine just roar along but the drive function slows. Also at that point it bogs down on even small hills. Is the transaxle completely sealed on these unts or can they be checked and fluids added? No visible fill points or gauges on the transaxle.
Replaced air filters Replaced fuel filter Changed Oil (no oil filter this model) Replaced and regapped plugs Replaced drive belt Complete physical cleaning All adjustments for forward, reverse, brakes made Completely disconnected brake function as a test in case pads expanding Tire pressure is fine All lube points properly lubed Carb adjusted Removed deck to reduce weight and even put a lighter person on it
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Check your owners manual to see if there's an adjustment for the clutch
David
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