I recently installed a an external coil and MSD cap. I runs fine, but the tach is jumpy under 3k and above there it's pretty much gone. Turning the headlights or brake lights on helps it work better. Kinda sounded like a ground problem so I checked and cleaned every ground, then added a bunch more. No change.

The only change was the external coil. I'm tryng to find a spare distributor I have to see what happens, but can't seem to find it since I moved

 

where do you have your tach wire hooked up at? or is it a factory tack built into the cluster

 

Stock tach. It ended up being the ignitor