I wouldn't think it would be a low oil issue and especially since it was only 1/5 qt. low. Some have said theirs have shut down due to the ambient temperature and pavement heat build up causing the generator to heat up and shut off.
Was it rather hot the days it was shutting off? If this wasn't the cause, I would suspect a fuel problem, i.e., trash or old gasoline. Some have sworn by Seafoam additive in helping to clear up some fuel issues with theirs. I've never tried it myself.
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