My tach and speedo have gone crazy. When I start to accelerate they seem to just climb at an insane rate and sometimes they jump around. When I put my foot down the tach goes right to 7k pretty much and I know it should only be at like 3-4k and it doesn't take much to get the speedo up really high either. When I stop/idle both seem to work fine. They also SEEM to work properly sometimes but maybe not. Any ideas what to check? Car is 1997 M6 w/OBDI if it matters.

 

I would suspect a loose ground or battery cable connection.

 

Yep, bad ground. Clean the PCM ground, near the passenger side strut tower.