1985 Dodge Truck Backfires/Stalls once a day

Engine Performance problem
1985 Dodge Truck 6 cyl Two Wheel Drive Automatic 200, XX miles

My 85 dodge d150 Backfires and Stalls once a day wether it needs it or not so to speak! Usually within first 15 min. Of opeartaion then it gets it out of its system and runs fine rest of day! Ive blown 3 mufflers in past year and found an ole john deeere tractor muffler strong enough to withstand the burst and havent blown one now in 3 months but still does the backfire/stall everyday like clock work!Timming been check ok, new dist. Wires, plugs, carb, timming chain etc. U name ive replaced it. HELP!