No a/c from Generac generator

I have a Generac NG66P generator in a 1998 National Tradewinds. It starts and runs consistently, but after about 30 to 40 minutes it stops generating electricity. The engine continues to run, and has a slight increase in RPM, but stops producing a/c. If we turn it off and restart it within 15 or 20 minutes it will still run well but not produce electricity. After waiting several hours, it will again produce electricity. It has been doing this fairly consistently for several months now. Once or twice it continued to produce electricity for over an hour and we shut it off while still functioning properly. We generally only use it to recharge the batteries. Doesn’t seem to matter if there is a load on it, as it produced a/c while running both air conditioners – but only for about half an hour before the a/c was no longer generated.

I have checked the brushes – they seem to be clean and move freely. I checked the generator wires at the transfer switch while it was producing electricity and got a reading of about 110 to 116 volts. When it stopped generating I checked the wires again and only got a reading of about 4 to 6 volts. I’ve downloaded copies of the Diagnostic Repair Manual 75239 and Technical Information Bulletin Y-788. Did all the tests that I could without having a frequency tester or an insulation resistance tester. May have to purchase those testers to do all of the troubleshooting described in the manuals.

I can’t figure out why it works so well for about half an hour and then stops generating electricity. Is something heating up enough to expand and cause a fault? Doesn’t seem like a loose connection would work and then fail so consistently. Anybody have any ideas before I pay a shop to check it? Hate to go that route because we’re fulltiimers – it might be hard to find a mobile tech who is familiar with Generac generators.

K&M