Z Force 60 problem
 

Hi all, I'm new to the forum and hope you can help me. I have a 2011 Zero Force Z 60.
After I mow for about 20 minutes, the blades slow down to the point where if I continue to mow, the machine will stall out.
I replaced the PTO clutch...still have the same problem.
Replaced the spark plugs...still have the same problem.
Made sure all the blades were turning freely and greased the mandrels.
PTO belt is not getting hot and has about 1/2" play.
When I turn the mower off, it will start right up again, immediately, but the condition remains the same as long as the engine is hot.
Any ideas?