stock tach troubles, help please!

Could be that the factory tach is just defective. You could probably find a used one on ebay for real cheap.

I don't know if i will be any help but i have the same type of problem. My stock tach started doing similar things. It will sometimes go counter-clock-wise, mostly at higher rpms. At idle, for the most part it stays normal but as soon as i throw a rev, it goes counter-clock-wise. Instead of trying to fix it, i just bought a aftermarket 5" tach, which works perfectly. But, it gets annoying to see the stock tach doing that all the time. Someone on stangnet told me that if i have a msd box, then that can cause the stock tach to work incorrectly. I don't curently have a msd box in, but had one and after thinking about it, the stock tach probably stopped working a little after installing the msd box. The guy told me I would need a tach adapter to make it work right. I'm not sure why it still doesnt work now without the msd in, the msd was in for may be a year. So, if you have an msd box or had one, that could be the problem.... Let me know though, if you figure out the issue. I'm tired of my stock tach going counter-clock-wise. OR IF ANYONE KNOWS WHAT ELSE IT COULD BE. Thanks!