Small Engines (Lawn Mowers, etc.): driv ebelt slipping, climbing a hill, idler pulleys

Question
i have a Craftsman tractor Mod# 917.276601 22hp. 42" deck, seems to slip when climbing a hill, with foot pedal pushed all the way , but picks up when i let off a little.would this be a drive belt?

Answer
Could be a belt, but sounds more like one of the idler pulleys is starting to go bad, I would remove the belt, and spin all of the pulleys by hand, listen and feel for one that is noisy and spins roughly.  Look at the belt and look for shiny/glazing.
Replace whatever you find bad.