Mowing cub cadet zforce loses power to blades

[ccquestion]

I have a 4-5 year old 44" cc zforce that runs great but loses power to the blades when going over heavy grass or leaves. The blades spin fine when going over cement or already cut grass but rapidly lose power and speed as they encounter resistance, even to the point of stopping entirely in heavier grass.

I know this sounds like a loose belt but the belts have been replaced, the tension arm and spindles/pulleys all check out fine and no one can figure out what is causing the problem. Could this be a issue with the PTO? Is there a way to check the PTO to determine if it is the cause without simply replacing the unit?

Any other areas i should check? Thanks for any advice anyone may have.