Tachometer Issues

There really isn't anything else that would cause that (well, maybe a bad ground, but most of your gauges should be messed up if that is it). I would think it is the tach that is at fault. It's a fairly simple system- the tach gets its pulse signal from the negative side of the coil (not sure in the case of the DIS coilpacks, but it would probably get it off the ignition module) and gets key on power to operate. There really isn't anything that would cause an improper signal to it from the ignition module unless the module or wiring harness is damaged, and I think you would have noticed that already.

If the new cluster wont fix it I would look at the harness going to each of your coil packs. I had one break and it only read 1/2 rpms

have the igntion module checked if its a Dual plug car, when the module starts to go bad, the tach can act funny.

Dr.