It sounds like the pulses are not going through consistently. I would check the rotor and sensor as has been suggested. There is a test that you can do to confirm this, but it involves batteries, resistors and volt meter. Not quite sure of the science of it all, but the volt meter measures the pulses by spinning the front wheel and taking readings from the meter to determine if the correct pulses are being picked up on the rotor.
Sorry, that wasn't very useful was it.
Where the hell did I put that Haines manual, sure it's in there somewhere.
