I KNOW I KNOW (jumping up and down waving hand)
the solder joints to one of the capacitors is getting bad,
here ya go
http://www.wrex-racing.com/docs/fd/Tach_Repair.pdf
if you don't feel like doing it yourself, remove the tach as per the instructions and take it to a local computer store that you can trust to be careful and have them do it. The capacitor is very obscure and the guy that's doing mine for me right now is still waiting on them (there are two he's replacing) to come in.