The rev counter signal wire is IG-, its a black, screened wire that comes from the coil and it can also be found at the diagnostics plug, on the side.
Its an electrical problem, its either a bad connection in one of the plugs the wire passes through or the tacho itself is bad in some way.
If it was me I would start by checking the back of the instrument cluster, find the IG- wire and have a look at the trace running from the socket on the back of the cluster up to the tacho itself. If that looks ok then get a multi meter and check the resistance to the diagnostic plug. If its below 0.2 ohms then the wires and connections are ok.
An auto spark with the right multi meter should be able to tell if the signal coming up the wire to the cluster is reading correctly as per the engine revs, if you are not sure about working with wires then this is your best bet.