Take it out of the car - make sure the body of the gauge doesn't touch the body of the car. Now connect the wires directly to the solenoid for +12v and to the body for earth. See what its doing now. if its acting up still - you have a faulty gauge.

Note that most modern gauges have separate earth for the internal light and the gauge - make sure you have both connected or atleast the correct one for the gauge.

Oh and a volt gauge should work fine from the fused side of the ignition switched part of the fuse box (atleast it does for me). Plus they are always advertised as such.

Crisis Over Guys...I Did all the Neg Earths behind the Binical and rewires the Volt Gauge. I also Re-charged the Battery after it dies..as I expected it to I also soaked all Batter Terninals in boiling water.
I didn't have to do much to the Body side as they were alread rubed back.....But all seems to be Perfect..Cheers Guys

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Stop Licking the Dog...I Don't Care Who Started It

more info-
Gafmo brought it over here yesterday and it read 13.5 volts on his V meter at idle. I hooked my multimeter to the same power feed (battery cable) and earth points as his meter, I read 14.0V, near enough. Flick the parkers on, mine still reads 14.0, his went south. Go figure....
Weird. But he then found he had a bad connection on the battery or it's earth point.
Here's hoping.....

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.