karettop

My Stang has a squirrelly tach. It will sometimes peg all the way around or just fall on its face. I can tap it and it will function for awhile. My question is where is the computer getting the signal for it. Anyone with the know would be appreciated. BTW, it is a yellow 98 GT that has be outfitted with a BBK CAI and 78mm TB and a MAGNA X conversion. It really snarls now!