95 riviera tach jumps around

hi i have a 1995 riviera non s/c, i was wondering if any one has encountered this problym or has any possible idea as to what may be causeing it. the tach on the my car works perfectly fine if the car is in either netural or reverse but in park or any of the drive gears the tach starts to jump all over. origionaly i thought this may be caused by a fault in the crank shaft positon sensor but after replaceing that and hooking the computer up to a live data scanner i realized that the computer is getting the correct signal from the sensor just the tach is going crazy. so naturaly my next step was to replace the instrament cluster.. still no cange.... any ideas?

thanks kyle