I just traded into a 2003 1/2 Cub cadet commercial m60-kw ZT mower with the 25 hp Kawasaki engine.
I was told it may need the deck straitened out as my sister in law ran over some cypress knees with it and bent the deck.
The mower runs great and drives like it should but when I cut grass at my lake the blades loose power when on a grade and go back to turning
on flat ground. I am not running the blades into the dirt or hanging them up on the inclines, they just act as though the PTO was turned off till I hit level ground again.I was thinking the belt may
be stretched or the tensioner is loose. My brother said he tried tightening the tensioner and that it was still throwing the belt although the belt hasnt come off for me since I turned the belt aroung to the correct side.
He had the belt turned inside out running the flat of the belt in the v grove of the pulleys which is why I am wondering if the belt is stretched out causing it to slip and not run the blades. Does anyone have any ideas what else if not these issues could cause this blade stalling problem?

If the belt was run backward for anytime I would change the belt. It isn't made to take the tight turns with the thick side in. If the motor stays the same rpm, it's in the drive portion. Check the fluid in the hydrostatic drive.

Good idea, I didnt think of checking the fluid.

If he hit something hard enough to bend the deck, the hanging brackets might be bent too.  That might make the belt looser then it's supposed to be.

Ill go check those now. thanks for the heads up.

OK looks like the hydraulic fluid is OK maybe an inch or so from the filler neck.
The deck hangers look OK as well.

There is three belts on that mower. A drive belt, mower belt and a pto belt. Have you checked the pto belt?  

If the PTO belt is the one that runs from the center of the deck to the engine, yes. If not, then no. I am guessing the one I checked and flipped around is not the PTO belt?