More than likely, your motor has internal damage. That'* exactly what my motor did right before i spun my #4 bearing which was 2 weeks after i had the LIM gasket replaced. The mechanic didn't change my oil. My car started running REALLY rough, RPM'* bouncing around, oil pressure bouncing from 0 to 60 the whole time. And the car would stall when you put it in gear. What'* happening in your motor is that there'* probably coolant mixed in with the oil and that coolant will make your oil gauge fluctuate alot. I hope you have a warranty or can afford a used motor. If your motor is doing that and you rev the motor, you should hear some knocking and it'* probably coming from the bottom of the motor where the bearings are. If you don't hear the knocking, you will in a day or 2 if you keep driving it like that.