I don't think it was ever resolved on this site, as I have the same problem too. I have looked at most of the different threads on this subject, but still can't get mine working. I might have to just spring for the cash and get a new tach. I'm starting to think it is a 403 problem. Most of the ones I have seen are late 78 on that folks have trouble with. Don't think many tach's on the Pontiac motors had problems. Let me know if you get yours working, and I will post what I did to get mine working, if I ever do.
Thanks.