if your brake lights work then your switches are ok, then you check your starter relay, bridge the 2 thick red wires out if it starts the relay is faulty, if not take your starter off and give it a 12v fed if it spins its ok, if not change it, lastly i have replaced shed loads off clocks for bad earth faults as the ignition system runs though them, you cannot fix them as they are sealed units, just get a second hand one push comes to shove the 125 versions will work but give an incorrectreading a give away is as you describe, rev counter bounces around and drops to zero then starts to work again!
m, April 2010
