Temp guage and electric gremlins

Hi all,
I have a strange issue with my Defender 300tdi
After I did the head gasket, the guage jumps straight to halfway up when you turn the ignition on from cold, I have since changed guage and both sensors and still the same?
I also have a surging alternator so the battery light flashes occasionally, could this cause it?

Have you a good earth to the engine. Jump lead one on to test.

I remember forgetting to connect the engine earth strap on my second Mini. It started, just, and when i was casting my eye over it had a pants filling moment when i noticed the return spring on the carbs was glowing like a heater element! I think it was earthing through the throttle cable!