Hi Lewis,
There's plenty of information on common problems in the "Hot Poop" section. I'd check the wiring is intact (if it's been incorrectly routed at some point the bolts on the brake disc can wear through it) and connectors (in the plastic box under the tank as well as those on the instrument panel) are making good contact before taking the wheel off.
Having said that, chances are it's the rotor in the wheel has the lugs broken off... that is the most common cause of this problem.
Rev counter... similar story. Check the connection to the throttle position sensor (TPS) at the right hand end of the carbs as well as those under the tank.
Cheers, James