It is hunting. You need to tune the generator. Refer to page 30 (section 6-10 Governor) then page 33 (section 6-13 Carburetor Mixture Screw Adjustments) of the service manual: www.manualslib.com/manual/833317/Onan-Microlite-2800-Series.html?page=30#manual. If it still does not run correctly, clean the carb. This is a constant RPM generator. So it'll go to the tuned RPM and stay there, shifting slightly when a load is applied.