I've done the following:
1) Took the coolant temp sensor wire off the sensor while the ignition was turned on. The needle dropped to the bottom of the scale.
2) I held the sensor wire against a grounded portion of the engine and the needle instantly went to the top of the scale.
3) I reconnected the sensor wire to the coolant temp sensor and pulled the ground wire from the underside of the coolant gauge. Next, I ran a separate wire from the battery ground to the ground connector on the gauge.
I still get that nervous, jittery behaviour when I rev the engine
Could it be a faulty coolant temp sensor?
Or could it be that I've overtightened the nut on the underside of the gauge? Would that result in such strange behaviour?