I just had a very similar situation in my 71 Charger R/T 440. Temp gauge went above hot when driving but looked OK at idle. You could see the temp gauge move a little when the thermostat opened but as soon as I started driving it would climb to over hot. A laser temp gun showed everything was good and there was about a 30-40 degree drop from the top hose to the bottom. We checked the gauge by unplugging it and then grounding it and the gauge worked fine. My friend decided to run a ground from the coil bracket to the body and surprise everything worked great. I removed the battery and harness ground on the radiator support, cleaned it and reassembled and everything works perfectly now.