Time for a new belt or tensioning spring.
Check that all of the belt keepers are in place.
Keepers that bolt on to the body oft fall off and you can continue to mow without noticing it.
replaced belt and changed oil in transmission, I believe all keepers are in place. what would cause it to slow down only when going downhill? runs great on flat ground or uphill, problem only occurs when going downhill.
Did you check both idler pulleys & bearings when you replaced drive belt? Post your 7 digit model number, example: 1692182. Bad bearings in idlers will cause belt drag as well as heat buildup & premature belt failure.
