As mentioned, the problem is most likely in the distributor, specifically the RPM output from the ICM, [ignition control module or igniter] it is the blue lead at the distributor assembly.

The first thing I would check is the connection of the blue lead to the ICMs RPM output terminal.
This is the ICM...

There are only three components to your Tach.
1. Ignitor
2. Wire
3. Gauge Cluster (which has already been changed)

I have a box of aftermarket dizzy's all bought brand new from Ebay...everyone has a problem, just outside the warranty. "Ye olde box of Honda dizzy's"