As stated above, check grounds for sure.

Also, if the tach was working, and isn't anymore, that is ran to the distributer, check the dist. cap, rotor, etc. Could be a coil aswell.

Ton's of things it could be, also take the vtec solenoid apart and clean the screen out, if the oil builds up in there and can't get through like it use to, it will cause problems.

And if all else fails, I like blue Si's, so PM me a price. HAHA