While I can not comment at this time on your current issue I can say this:
At one time I purchased a 1,000 watt portable.. When I first used it several devices "Complained" about the quality of the power.. Ran several tests including a 'scope... Well, finally I just hooked up a light (300 watt flood light) pulled the cord and let it run out of gas.
After that all was happy.. Seems sitting in the box on the shelf some components had changed value.. use restored them (Electrolytic capacitors do this, well known "Feature" and as an Electroincs techinician I know this.. or should).
Every month, run Generator for at least 1/2 to 1 hour, under at least 1/2 load. This keeps those parts happy, cleans slip rings, keeps engine properly lubricated and helps prevent carb issues as well.
Or as my Mechanic said "You now have an orange engine" (painted orange).. My Reply was "orange on outside ok, orange on inside (rust is orange) bad.. (It is bright and shiny on inside.. runs good too).