stock tach help

The stock tach in my gt works but will sometimes go backwards. It has been like this for a while now, instead of figuring out how to fix it I just bought a new autometer tach with shift light which works perfectly. It gets annoying to see the stock tach needle going the wrong way between shifts even with the other tach. I think it happens more at higher rpm shifts becuase when im shifting slowly between first and second, the needle sometimes works right. At idle it works, but if I throw revs, the needle will go backwards under the one. I wanted to see if anyone has any ideas on what this is caused by or has experienced the same problem before I open up the dash. I have been thinking about just disconnecting the stock tach if it can't be fixed. I know it worked when I first got the car a few years ago and then one day I noticed it working incorrectly without any mods, etc. Any ideas?

I had a msd ignition box a while back, I took it out over a year ago. But that is very possible, it may have started after that was in.... So the msd screwed it up? How come it still does it without the msd in though? While we are on the subject of msd ignition..... When i do my ignition again, you think I should stay with msd or go with mallory or one of the other companies. A msd box is pointless when everything is stock right, one reason why i took it out.

Also, what tach adapter should i be looking for(part number) I have an autometer tach in there that works perfectly, I just get annoyed with the stock tach needle going the wrong way most of the time. Will a tach adapter change anything with the autometer that is connected already. And, the msd box has been out for more than a year, so the stock tach will always have problems?