I wanted to post a solution to a problem in case anyone else is having the same situation. I had a niggling problem with my tach in my 83 Rx7 (completely stock). It would jump around a bit when I was cruising...but under acceleration it would register normally. I put new spark plugs, wires, and a new distributor cap and rotor into it...and the problem went away. I know that there have been a lot of posts about tachs and trailing ignitors, but I wanted owners to know that you can replace the cap and the rotor and also get resolution on a tachometer problem.