rev counter fault code

[fletch]

You've probably gone over this before,but i tried searching and couldn't find what i was looking for.Basicaly my rev counter is playing up with the ignition on everything seems fine.When i start up everything is fine for 5 seconds then the needle drops for a second then shoots up to 7k then starts working for 5 seconds then the cycle repeats i've checked the exup valve everything is free that end and i have not noticed any drop in performance.So for the love of god please help me it's driving me insane  :'(