back fire

[Old Guy with 87 GT]

my car is doing the exact same thing ........in my case i'm pretty sure its from bent or sticking valves

.....the backfiring/missing started from a higher rpm and started to progress torward the lower rpm range the more i drove it .........it misses at idle and carrys on missing/backfiring the more i put the engine under load....i'm pretty sure with mine its bent valves becouse one the heads thats on mine isn't properly set up for my combo (missmatched valve springs ..long story).......after reading the symptoms you described i'd look into it ....mine does allmost the same thing you described