I am having an occaissonal jumpy tach reading and at the same time a slight engine surge at about 70 mpm freeway speed in my '86 2.8L. Radio noise too. I remember having this a few years ago and I thought changing the spark plug wires fixed it. I changed them yesterday and I still get the same behavior. I thought it might be the tach filter, but would that affect engine performance?
How can I fix this?

Thanks in advance

Going by your clues of radio noise and engine surge, I would suspect some sort of high voltage leak. Check your spark plug wires at night and see if you get a light show, especially near the spark plug boots. If you do see the sparks flying it's time for new wires.