John Deere 210 PTO won't disengage

I have a '74 JD 210 Garden Tractor that is in excellent condition and runs like a top.
My problem is that the PTO won't disengage after the motor is warmed up. I use it for mowing, this presents a few safety issues, and makes it difficult to start with the blades engaged. After the motor cools down the clutch assembly pops free.
Has anyone else encountered anything like this, is there something I can do or adjust to correct the problem? Or is something in need of replacement. I've adjusted the belt tension and that didn't help.
Thanks for your help.
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