Onan 5500 Marquis Gold - Stops running

We are having a very similar problem right now. The difference is that the generator will start fine, will run under load for approx 20-30 minutes and then quit. If I turn off the generator breakers so there is no load, it does not quit. Just had the fuel, oil and air filters and oil changed on the unit. Any suggestions would be greatly appreciated, we're trying to use it this weekend.
Info on unit: Onan 5500 Gold, Serial #L060998054, Model #5-5HGJAB-1038D Spec D, 50 amp