If the engine speed were actually jumping around from 3k up to 6k or 7k, it would be so obvious to feel you wouldn't need a tachometer. So I bet it's just the tach jumping around, right?

If so, flushing transmissions & cleaning injectors isn't gonna deal with the real problem. Wiring problem or loose connections SOMEWHERE at your distributor or tachometer or somewhere in between.

Got a multi-meter & a wiring drawing for your car? That's what you need.