Well my aftermarket tach gets its info from the negative side of the ignition coil. But I think the tachs that come stock take the signal from the computer.

Anyway you might want to check the frame to engine grounds. Or maybe change the wiring to go strait to the coil. Or maybe just swap out the tach to an aftermarket gauge in the dash.

Below is the PDF installation manual for my tach. ItÆs probably not very helpful, but anyway.

http://www.sw-performance.com/downloadfiles/15.PDF

Steve C