My best guesses have all been mentioned, timing, cam, worn distributor parts and timing chain issues. However it sounds as if it's leaning itself out at higher speeds. Perhaps the carb or fuel system is in question? I know an ice cold engine with a broken choke will backfire nicely through the carb, on a frosty morning. Also see if there is any slop in the distributor main shaft. If it's moving around that will cause the points to bounce at higher speeds. Good Luck.