intermitant AC power from generac Q-55G

Tnx for that info. Glad you got it to work.
Yes, you have to hold the start button as per my owners manual in my 2018.

It turned out to be the regulator. In the process I also learned that the new regulator is wired a little different internally. My old regulator would create 120V as soon as it started, The new one requires that you hold the start button 3-4 seconds even if the motor starts immediately to build exciter voltage. It took several phone calls and several hand fulls of pulled out hair to get that figured out. Hope that tid bit helps someone in the future. Thanks again for everyone's input.

Thanks for the good advice.I never load the generator before warm up but had not heard of unloading before shutdown.I do exercise it under load about ever 4-6 weeks. Have a lot of storm clean up to do but hope to get back to the problem soon.

Make sure you use a good multimeter like a Fluke.
I agree, check the wires, sometimes they have breaks hidden by the insulation . Don't forget to check the grounds as well as neutrals. Wiggle them throughout the wire where you can. Check solder at the circuit board for cracks. Check the wires at the junction box.

Do you normally turn the generator off as you're using it with a draw on it ?
Ex: is the a/c on or other electrical items when you turn it off ?
If so, I was advised not to do that and get in the practice of turning off all draws prior to turning off the generator . Leaving things on can cause premature voltage regulator failures.
If you exhausted wire and connection testing , it might be worth it to just change the regulator and take from there.

As gbopp says, this could very well be a loose electrical connection somewhere. Check also the connections at the transfer switch (assuming you have one) or the generator receptacle for the shore power cord (if by chance you don't have a transfer switch). A failing transfer switch could also work intermittently even if the connections to it are tight.

I would tend to doubt it's problems inherent in the generator itself, as the generator control module typically will shut down the generator if there's a fault causing no output. (I know that's true for Onan RV generators, but only suspect it's also true for Generac ones...I could be wrong there.)

Are all the connections to the generator clean (corrosion free) and tight?
Reset the breakers on the generator. Turn off and back on.
I know the Onan in our 96 Southwind will act strange if there is a little corrosion.
Do you run it under a load on a regular basis?