I know this is an old thread, but I've got a similar issue with my Generac NP-66G. Mine has been giving me issues for the past week or so, and now is to the point where it reliably stops generating power within 15 minutes of start. When that happens, the engine bogs down as if under heavy load, the output reading goes up to 160V at 59hertz, and a GFI outlet in the vanity trips just before the EMS kicks in and disconnects the generator. We've occasionally had the problem with the GFI outlet for the past year or two, I just never put 2 and 2 together to see it as having anything to do with the genset.
This is more or less consistent at this point. So, I'm just confirming that I'm also having the same issue with the voltage putput going up to 160V before CB3 apparently opens. Sometimes power will come back for another minute or two before it does the same thing. The engine has always ran perfectly, whether under load or not.
