Unplug the small connector on the back of the generator, then see if the stalling occurs. If it does not, have the generator professionally load-tested. If it can only deliver one third of its rated output current, it has a defective diode. "Ripple" will also be very high. That will also cause it to develop much larger voltage spikes than normal. Those spikes can destroy the internal voltage regulator and diodes, and they interfere with engine sensor signals.