We are describing the same thing.
It is a wire problem.
It may be at primarilly 4 different locations:
* instrument housing
It may be due to broken/burnt wire. Damage isolation of wire shortring it out. Loose connections.
¤ Joker - faulty rev-gauge
The most likely cause is at the coils, less the instrument housing has been opened... Of cause it is well known that the wireloome gets "worn" as it passes the steeringhead resulting in internal shortings - on the older beemers i.e. However, if I recall correctly, Samis bike is a 1992 GS - if so, his bike does not have the electrical connections inside the drivinglight but under the tank on the frame. Hence Samis problem is most likely to occure at the coils and their connections (he should have the newer 12V dual-coil and not the old double 6V coils).
So my first guess is a lose connection, i.e. the ground connection. My second guess is a damage wire (the gound wire as it enters the wireloome).