86 GT problem.

[mg]

I recently acquired a 1986 GT in great shape, but it's developing a problem. Sometimes it runs fine and the tach looks normal, and sometimes the indicator bounces wildly. When the tach is flipping out the car idles too low (sometimes dies) and doesn't run as well. Then the problem goes away, but it has become more persistent. It has 124 K miles on it and is a pretty fun car. Anyone have any ideas. Thanks very much. mg