I put in an electric oil pressure and water temp gauge in about 4 weeks ago and had the same problem with the water temp gauge. I spent 2 hours checking all of the wires and calling Auto Meter tech support. They figured I had a bad gauge. Well, after checking all of the wires I decided to run a new wire from the sender to the gauge and see if that would fix it. You guessed it, that was the problem. Somewhere along the line, the wire must have become cut and was grounding out. Works fine now. So long story short, check your wire to the sender. Hope this helps.
Hoosierboy