Onan frequency is directly correlated to engine speed (rpm's) - suggest you focus on why the engine is turning itself off.
Two common reasons for generators to quit after you release start switch are defective oil pressure sensor or failure to produce electricity. I believe you can jump the oil pressure sensor to eliminate that as a cause. One of the common reasons that generators run for a while and suddenly quit are fuel related - weak/defective fuel pump, fuel filter, or even tiny hole in fuel line.
