Daniel,
There is a tach tape somewhere on the forum. John Will posted it sometime ago. Maybe if you do a search for tach tape you can find it. I agree with John, the most probable cause is in the tach tape or reader. When reassembling the tach sensor make sue you have the recommended clearance. Try this first.
Did you say the engine speeds up erratically in conventional mode? If that is the case, the problem could be different.
bruce
