My previous Onan 5500 would do that. Turns out that it was only running on one cylinder due to a faulty ignition. It would handle a little load but not much.
Try just revving it without a load by moving the throttle linkage, turn off the circuit breakers first though so you don't damage any electronics. That would at tell you if the motor is running right. If that check's out then sounds like the governor is not reacting to the added load.