It's doing a dance and I don't like it. I'm assuming it's a grounding problem either at the ECU or on the tranny/engine. But any ideas to exact it? it doesn't do it all the time, it bounces up everyonce in a while a few grand and stays. until I I get going hard and it goes ack down.

 

I had the same problem. This may sound stupid but for me it was a connection between my aftermarket tach and a metal spot on the car. When the metal touched the bare connector the tach would dance. Might not be the problem but it is an easy fix if it is.