Missing

1985 12A with random miss. I've replaced a lot of parts with no improvement. Misses at idle or at any highway speed. Only time it runs smooth is when accelerating. It can be a little acceleration or full throttle. Has plenty of power and has always run great. It was missing when I bought it 3 years ago and I still haven't found the problem. I could tell that someone was looking for it before I bought it. Thank you for any help.