[Phil]

Veek
Try a good ground as Johnathans mentioned, and also look into a tach filter. This was used on some of the early GM cars with fuel injection to filter out stray pulses that were affecting the tach.
I would bet that your ground is bad or there is a poor connection causing the tach to find ground through one of the other accessories. Another thought is maybe the tach isn't getting enough voltage because of a poor connection, and this will cause crazy readings. I recently had an alternator go bad and just before the car stalled because of the lower voltage the tach and speedo went full scale.
Hope some of this helps
Cheers
Phil