What would make the rpm gage jump up and down while the car is in motion, and the engine light is on. I might add that when the rpm is jumping I can feel the car softly ****. I own a 2002 Chevy Malibu, I just had a belt and a harmonic balancer replaced. Please note that this was not happening before the belt and balancer were replaced.

Optics - Gentex - 2002 Chevy Malibu - All Malibu models - K5 Auto-Dimming Rear...