ok i have dealt with this one many-a-times. I have the same year and make car you do.
a few solutions
Under the hood, on ur driver's side fender, is a little black cylinder called the Tach Amp. 90% of the time its the culprit.
1) unscrew the tach amp. pop out the inside thing on the cylinder. Take a cigarette filter or some small cardboard wedge it up in there, put the silicon topped thing back into the cylinder and fasten it back to the socket. This might fix it for you for a little while, it's not permanent thou.
2) find a junkyard and swipe a few of them
3) buy a new one. Part Number: 001 545 59 32 (check with ur part to make sure thats right) that will cost about $70 from ********
www.*************** ( I just did this after going throu 2 used)
Kris