I had a similar sounding prob recently and it turned out to be the distributor.
Just cruising in traffic at about 40 the car started misfiring badly, first thoughts were that something had gone badly wrong with the timing / valve gear or something. Really sounded rough, like running on 2 pots.

After a load of timing checks and head scratching it ended up being the 3 pin pick up plug socket on top of the dizzy. It had become loose and the wires were bare and frayed inside.

New dizzy and away it went