On and off rev counter

Hello everyone,

I was wondering if anyone could help me or have ever experienced the same as me. I have a Fabia MKI VRS with 60 on the clock.

My Rev counter works on cold days but is very jumpy. However if it is say 8c plus the dial either sits still, goes up to idle and then doesn't move or sometimes if I Rev it it will take the end of the Rev to be zero, for example uses 4 as 0.

I have thought maybe it is the crankshaft sensor but I am no mechanic so I don't have a foggiest.

Please can someone help me?

Thank you very much,

Caleb.