Hmmm... The last time that my car started missing and backfiring was when the distributor was starting to go. The bearings were starting to wear and it would sometimes cause a bit of wobble that would make the rotor miss the contacts on the distributor cap. Do you ever notice any high pitch squealing or metal scraping sounds from that area of the engine bay?
The only other thing that I can think of is the possibility that something is slightly warped from the overheating, resulting in the same sort of effect at certain RPM's.
The only other thing that I can think of is the possibility that something is slightly warped from the overheating, resulting in the same sort of effect at certain RPM's.