I just found one post about this problem, finally! It says the same thing that i said, that since the tachometer gets it's reading from the ignition coil ($83), it could be that. here's the link.

http://autorepair.about.com/library/faqs/bl830.htm
One more post about the same thing with someone that actually had the ignition coil replaced.

http://autorepair.about.com/library/faqs/bl041b.htm
if i find more, i'll post.
hope you found this helpful as i know i could used this info myself.