Iirc the rev counter is electronic so can only be the rev counter or wiring to it/from where signal is picked up(ecu I believe) so that's where to start your search for broken wires etc. take the clocks out nearly all the way and check plugs on back of clocks (could be a dry joint and a jiggle could sort the problem) if still no luck I have a rev counter here from a set I bought when I needed a fuel gauge. Hope that helps