Ummm... the 6th gens still had a distributor, right guys?

Assuming so I'd take the cap off and make sure the connection underneath are nice and clean and if that doesn't help I'd guess that the coil is going bad... or maybe the ignitor.

Just a guess, could be the tach. itself :shrug:

Does the tach. needle ever jump at all while driving or read normally for a brief moment? If it's constantly off w/ no jumping etc., plus the fact that it's reading under, not over, it seems more likely that it might just be the tachometer. You've never removed the needles have you?