So the problem has to be something in the power supplied by the generator that is being sensed as a ground fault by the GFCI in the plug. Some kind of interference. And something in the generator changes after some period of operation since the tools will operate successfully for the initial period.
My next step would be to either try to add some kind of filter to the generator output, or get another new generator and oscilloscope and look at the output of old vs new.
My next step would be to either try to add some kind of filter to the generator output, or get another new generator and oscilloscope and look at the output of old vs new.