They were famous for ignition boxes going bad. For the longest time i had a spare box bolted beside the regular one on the fender well, when one started to act like yours i'd plug the other in and go on my way. After a while they seemed to get the boxes right and i no longer keep a spare. I had (still do) a '77, mid eighties, and mid nineties ford trucks and i think they all took the same box. I recall useing the same box on the '77 and the mid nineties truck, i used them both for towing (460 motors) and couldn't afford to have either of them break down.
Anyway, i'd be replacing the box. Esp if you drive along, feel the power go away and comes back on with a ferocious back fire - you don't let that happen twice.