cowger, you reported reading 62hz. after you replaced the AVR, was that with the generator loaded, covers and lid all on and closed? Slip ring and brushes will have no effect on 'overspeed' faults. Most overspeed issues with your model are due to the ignition magnetos, the control gets it's speed signal from the mags and also due to the age of your unit the mags tend to break down and cause starting problems. With that said, I replaced the mags in one that was repeatedly giving the overspeed fault, a few weeks later it again faulted on overspeed, replaced the controller and it's been good.
Thanks, buckshot. That 62 Hz was no load, most likely with the circuit board cover off and the lid open. Does the lid open/closed make a difference?
Good to know that I can forget about the brushes / slip rings and work on other possible causes. I'm willing to try replacing the magnetos -- just to be sure, you're talking about the item labeled "ASSEMBLY, IGNITION COIL" in the manual, correct?