Rev Counter fail

[Garlicbread]

Basically my rev counter operates when it wants to, it sometimes completely stops working while on the move but only seems to happen under 2000rpm
it doesnt usually work cold start first thing in the morning, it also starts playing up if i have no throttle while going down a hill and the engine gets cold (like 70 degrees temp reading)

i dont know whether it needs some increased voltage over normal for it to work like its encountering resistance??
if you can tell i dont do electrics, yet i buy a french car.....

any help would be awesome!