Tractor Repair: Rear PTO kicks out of gear, double clutch, cylinder diesel

Question
First, Thank you for helping! I have a 1989 Ford 1520 Model #AH4137 compact 3 cylinder diesel 23hp tractor.It has 870 hours.I can put the rear PTO in gear without an attachment, and it rotates and stays in gear at all rpm. When the rear mower, or a load is attached, it is fine at low rpm but above 300 rpm it jumps out of gear. I tried to hold it one time but it kicks out. I checked the rear shaft for any play and there is no play. The mid PTO is ok also.Since it works fine with no load and sounds quiet I am figuring it is not gear related and could be clutch related but I am a greenhorn to Tractor diagnostics.It is a live pto so there is a double clutch.I could use your advice on what I need to inspect,adjust or overhaul.I am an above average mechanic and have an overhaul manual for this tractor.Any advice you could donate would be greatly appreciated.
Thanks again,
Tony

Answer
Hello,

  First, check the pto lever and linkage to make sure it is adjusted correctly so that the pto is completely engaged before the lever hits the end of the slot (if it has one).   You should feel the lever click into position.   This problem would not be clutch related.  It is possible that the internal shift fork is bent or broken.  There could also be gear problem even though it doesn't make noise.