Sounds like a cooling problem. Are you checking coolant level at rad pressure cap, rather than just visibly looking at overflow tank?
The cap is under a small flap, just above control panel of gen; one screw holds it in place. Check the cooling fan blower belt, make sure lots of cooling air coming out at the bottom of set. The end panel of this set is usualiy easy to remove, take a dozen or so 1/4 bolts out.Then you can visibly check belts, etc. The oil pressure switch is readily accessible, in cyl head near front of head. The hi water temp switch not so...it is around on other side of head near thermostat. The control panel also has a overspeed shutdown function, very sensitive, can be adjusted, if you remove odd shaped plate on left side of gen, gives you access to control board and voltage regulator