I've seen a bad coil wire cause this many times, but it could also be the tach itself. As you have some movement, I would not think that there is a connection problem. If you pull the panel behind the cluster, in the round wiring connector that goes to the cluster itself, there will be a white or a white/blue wire that sits in the #12 position in the plug.(I think that one 1 side of the wiring connector it is blank, and on the other side the wire is red) You could check the signal there with a remote tach to confirm its pressence. I remember it as I needed to add a signal conditioner when I added my MSD, and I did it under the dash/cluster, rather then in the engine compartment.
Kary