Facts: Mustang Coupe 1965, 302 engine. Water temperature gauge jumps over 100% seconds after i start the engine. I tried a different 100% working gauge and the result is the same, so gauge is not the problem. Wires are all new. I have replaced temp. sender unit with a new one, and problem is still the same.

Engine works fine and doesn't overheat.

Any ideas what could be a problem?

 

Sounds like a short to ground somewhere. Does the gauge go down if you disconnect the sending unit?
If you have a meter check for continuity between the sending unit wire and ground. If you have continuity, then somewhere you have a short to ground.