Question about water temp indication on a Ford 302/306:

My water temp gauge used to work fine but now it is either pegged at the left or at the right.

I have the Autometer gauges that came with the kit and a Holly Sniper 4150 EFI with the handheld touchscreen controller. When I start the engine the water temp is pegged at the low end (100°) and doesn't rise as the engine warms up but the Sniper's controller display shows the water temp steadily increasing. At about 190° on the controller, the Autometer gauge pegs out at the high end (250°). As the temp decreases below 185-190 the gauge goes back to the low end.

When initially filling the coolant I didn't know to raise the front end so there may have been air in the line. So today I removed the radiator cap, raised the front about a foot and ran the engine. I did see some air bubbles and the thermostat seemed to open/close as it's supposed to. The Sniper temp indication seems to indicate everything is working properly except for the gauge...it's either pegged to the left or to the right.

The Sniper's temp sensor is in a port on the top of the manifold, and the gauge's sensor is right next to the thermostat. I do have the gauge sensor grounded on the thermostat housing...could that be the problem, or something else? Should I move the sensor to another port?

Thanks