generator overheat, any ideas??

Scott thanks for all the info.. I have been doing a lot of studying on this here interweb and that's about what I have come up with... There is a Cummins dealer right down the road from work, so when I get off I'm buying a thermostat and a sensor.. The one thing I would like someone to tell me though, when they start there genie with the radiator cap off does coolant shoot back out of the little opening in the neck that runs to the top of the radiator? I've never ran it with the cap off.. You know like in a pickup or car you could open the cap with the motor running and add coolant. There is no way you are going to do that with what mine is doing.. I just wonder if that is normal?? After all this if that doesn't work the only other thing I know to do is pull the radiator and take it and get it cleaned out.