Tempriture gauge

Gday guys!
Really hoping you can help me with this problem Iv had with my tempriture gauge for the last 18 months.
It has two issues.
1. Sometimes it'll be sitting in the normal range then jump strait up to hot. It also occasionally does this when I first start it in the morning when the car hasn't even warmed up. To fix it I usually bang on the top of the dash and most of the time it comes back down. I pulled the cluster out the other day and waited for the gauge to play up. When it did I jiggled the multi pin connector and it went strait back down. I had a bit of a play with it but it doesn't seem to be loose or anything like that. What do you think I should do?
2. When the car has heated up to normal, if I turn the headlights on, the gauge goes up a little. High beams, goes up a little more, a/c plus high beams and its in the red. Turn them all off and it goes strait back down to normal. So I'm thinking it's an earth issue but I don't know where to start. Where is the earth connection for the dash?
I did take it to an auto elec here in Alice Springs but after having it booked in for a week, the day I took it to him he had "forgotten" to write the booking down and said he could spend a quick half an hour on it. After that he tried to tell me I need to replace the radiator to fix the issues. Now I am pretty sure that the radiator has nothing to do with the issues. Extremely frustrated as I live four hours out of Alice and took a day off work to get it to him!
Hope you guys can help me!

so sounds like an earth problem,but not speaking from experience....but that where i would start if no one chirps in

It does sound like an earth problem.
I just recently was having similar problems with my EGT gauge, the gauge was allover the place and flickering.
I already had an extra earth cable from the body to the chassis, I put another cable from the motor to the body, no more problems.

with my old GQ,
I added a heap or extra Earth Cables, and never had any more electrical gremlins
I used standard gauge Battery Earth Cables

main battery to chassis
main battery to Motor
aux battery to chassis
Gearbox to chassis
body (engine bay) to chassis
body ( rear) to chassis
Motor to chassis
rear diff to chassis

I went over board, but it sure worked

Thanks for the advice so far.
I'm hoping someone can point me in the right direction in finding where the earth runs to for the dash.
I'm really not too sure what to do about the plug on the back of the dash. Has anyone got a wiring diagram so I can work out which one runs the temp gauge.

Bump, anyone?

In the gq from memory the are on the sides of the dash into the A pillars

Mines a gu series 3