Thought I was having the Tic-Toc-Tach / HEI blues because my tach needle would fly over to the high-rpm end when I just stepped on the gas a little bit.

Well it turns out that the Tach wire that goes to my Accel HEI Cap/Coil was too close to a spark plug wire. I simply moved the tach wire away from the spark plug wire and presto...tach reads perfectly now through the entire RPM range with no needle flutter.
The same kind of deal is probably the case if any of the cars accesories make the tach needle behave strangely. I have seen other cars that would do this when the turn signals blink. Enjoy.