With all the constants in a closed/open system, I had the same issue. Engine would start up run normal for a set time and shut down. I had a trash, old water pump in an elbow of the coolant system. I cleaned that out after I replaced all the water pump (raw water) circuit. Note: a short time later the engine ran funny, surging and not leveling out, that ended up being a short in the generator that was causing lots of load under a no load condition. Ended up buying a newer generator.