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MotoGP God
Bikes: 748S, HM1100S, V4SP, Was: DD-A #111
Posts: 13,088
Join Date: Feb 2002
Location: Back in Essex
Mood: Passion Killer
Sounds like a bad earth connection to me - the rev counter works off pulses and any noise (ie, short circuits) causes it to jump about.
Check the main connectors by pulling them aparts, inspecting and cleaning, then reseating. Have you taken the battery off recently, to fit an optimate (charging) cable or something? Make sure the connections are tight.