Outer Limits 4x4 Board

Okay, here's a good one. I have an Autometer Pyrometer 3344M installed in my 1990 HZJ75

I took 12v from the ignition wire to the aux fuel guage on top of the dash. Earth was what I believed to be earth from the same location.
The needle now jumps around SOMETIMES with the indicators, SOMETIMES with the LED bar and SOMETIMES with the radio. Othertimes its okay. I moved the earth to the dash (metal) and the problem remained. I have had the guage tested by Rocket Ind. (where I bought it). I have had the wiring inspected by an auto electrician who said it was wired correctly (I don't think he looked very hard as it the dash looks fairly untouched and he only charged me $80). I have run a wire directly from the dash to the guage and it still jumps around.

Apart from drink heavily, what do I do now?

TIA.

Seems to only be happy when earthed directly to the battery. Doesn't matter which bolt, screw or other metal part, it all seems to have the same issue. I'll clean the body earth wire but I can't see an issue with it especially as this is the only thing causing greif. Positive supply seems good.

So I've cleaned the earth from the body to the battery and all is well. I have also located the earth wire I should have picked up originally. I will upgrade the main body earth to something larger and more heavy duty.
I knew it was an earth issue from day 1 but couldn't put my finger on it. Dissapointing that an auto electrician couldn't find that though.