My Onan 4KW tends to race when I turn off the air conditioner, then comes down. Every once in a while it shuts down when I turn off the air. I checked the voltage and with out load it's 119-120. With the air on, it's 117-118 and when I shut off the air the volts jump to 122 and then after 5 seconds drops back to 119-120. Is this normal or is it out of adjustment. The Hz is around 59.7 to 59.9.