The usual problem is a bad ground at the gauge itself, easily remedied with a separate ground wire from the backside of the gauge--there's an attaching nut there that's a convenient grounding terminal--to the car's body.

If that doesn't cure the problem it's time to mess with the sender unit itself. Sometimes it's a bad ground there, other times, you'll have to dismantle the sender unit itself. But if your gauge works intermittently, it's more likely to be a bad ground than a broken sender.

cheers
mike
_________________
'69 Nevada sunroof--"Wolfgang"--bought new
'73 Sahara sunroof-- "Ludwig"--mine since '78
'87 eta E30 coupe--"Johannes"
+ Fiat Topolini (Benito & Luigi), Renault 4CVs (Anatole, Lucky Pierre, Brigette)...
and Kermit, the Bugeye Sprite