E30 Zone

ok had massive issues with the cooling system since the last service.
im not sure weather my gauge is working correctly.
I will be driving along and it will say its starting to get warm (one notch above halfway) so i pull over stop and restart the engine then it will go to halfway and stay there for ages. the temp gauge needel appears to wobble slightly and will wobble alot with the sub on. also it will go up to nearly hot then just sudenly drop back down to just over half.
is the gauge broken, do i have a bad earth or something.
very confused any help greatly recived

try the nut thats on the back of the clocks if its there at all just nip it up if its come loose

oo sorry forgot to mention tightened the nut up also the gauge reads nothing when the brown plug is pulled out and reads full when grounded to earth

help could this be a bad earth