Had a similar thing happen on several cars back in the 80's that had HEI. Would run great and then run crappy. I would take the cap and rotor off to look at the module or change the module. All would be great for a while then it repeated. Eventually every car that did this died. On one, I started looking at the rotor....under the contact for the center, there was a tiny hole eroded. There was some faint carbon tracking thru the hole that grounded the rotor to the distributor shaft thru this hole.....it seemed to be heat related to the conductivity... and related to me removing the rotor to change the module....I would destroy the carbon path and it took a drive to re-establish it. I also found that cheap crap rotors were the worst, real Delco rotors (the whitish ones) were the least susceptible to the problem. I talked to a couple of shops who admitted to making quite a profit by changing rotors but charging for a module. Also, look at the cap inside for tracking....it is really faint.
Mike in Tucson