[undy]

It sure sounds like you have an intermittant loss of spark problem. That could account for the tach needle fluctuations too. As you said, it could be the coil, distributor cap or your distributor pick-up. I had a a Mallory uni-light many years ago and can remember having to wipe off the distributor's pick-up light every now and then due to ozone deposits. It would cause it to miss if the lens got cloudy.