fzrer

I have a 1994 FZR600R Genesis. Its an USA import model with just 18K on the clock. All seems well with the bike with no fall off in performance or reliability. However, very recently the rev. counter has started to behave oddly though I've been informed this is some form of fault reporting. The counter flicks between 8000 rpm and 0 rpm, hovering around these for a few seconds. It did this very rarely at first but since the tanks been off as part of a service its happening all the time though the counter behaves normally for a short spell when the bikes first started. I ask; is this a fault code? What does this one indicate? What are the other fault codes? etc.