friend had JD tractor, low hours, all she did was shredding, she had same problem, don't recall tractor model.
Turned out to be there is a VERY SENSITIVE clutch adjustment for the PTO.....there is a turnbuckle on the shaft from the PTO engaging lever to the lever going into the clutch/transmission.
I tightened that turnbuckle by a couple of turns.... clutch slip went away....shredder began cutting grass again, no further problem in almost 8 months...
Net..I suggest strongly you investigate for a PTO clutch adjustment and see if tightening it helps...
This was about a 10 minute job, simple tools. Seems to me that there was a description about how to do this in owner's manual.