01LexGS300

Just bought it yesterday off CL. 169,000 miles. I had the car idling while it was in neutral today and the idle started fluctuating on its own. It would rev up and down to different RPM's. When this started the temp gauge also stopped working and dropped down to the C. It was at normal operating temp when this started happening. Also seems to idle slightly low when in gear (Drive), looks to be at around 500 RPM. No CEL is on. Engine runs like new otherwise. What could this be???