I am running Mallory Unilite ignition modules with Gaffrig tachs. The starboard ignition module was replaced last year, following the replacement the tach jumps around in idle and steadies out as the RPM increases (dead on above about 2000). I have swapped the ignition module to the port engine with the trouble following the module (port jumps around, starboard rock solid). I swapped the modules back and the problem again followed back. I have since replaced the ignition module and the problem persists. The tach will jump with any engine running condition (i.e. starboard running, port off, both running or the opposite when the module was swapped). Any thoughts would be greatly appreciated I'm at a loss.

alot of times a dirty/bad cap or rotor will cause a tach to act up that would be the first place I would look

I've seen the coil cause this more than once.

I had this problem with a bad connection in the engine harness plug.

Both caps and rotors were replaced this year. Wiring has been verified to be in good order. I don't understand why the problem would follow the module leading me to believe a problem with the module and replacing it and the problem persisting.