Running thermal grease, even got some fancy stuff by the time I got to the 3rd ICM.

I checked the ICM wires per shbox's diagrams, but who knows - won't hurt to check them again.

Here's a video of the tach:
https://drive.google.com/open?id=0B_...kpNbTdsTWc3N1k

I let the car idle in the garage for about an hour with a new ICM and alternator. Over that time the tach 'jumpiness' got worse. Alternator voltage dropped from 14.5 to 13.5 on average. Now I'm suspecting the opti/plug wires. Maybe high voltage leakage is escaping to ground some where and polluting the system and damaging solid state electronics. I'll try to get a hold of a set of wires (I'm on a first name basis with all the local car part shops lately...) and see if that makes a difference.

Thanks for all the responses, hopefully I'll get to the bottom of this soon. I'll report back with results!

- Josh