98 chevy k1500 missing

[67 chop top]

hello everbody im having a little problem with my truck. i cranks and runs fine but when i start to pull from stop it has alittle miss then quits till it goes into overdrive then it will miss when i just crack throttle easing up a hill or slite grade. it is magnified even more when i pull a trailer. i have replace iac valve, throttle position sensor, both temp sensors, done a complete tune up. ihad problem with leaking head gasket. so i pulled it down put new gasket seated valves and new seals, while it was down i installed anew fuel pump. all filters are replaced at oil changes every 3000 miles. i have checked for vacuum leaks but havent found any. checked air intake hose for leaks. miss doesnt start untill engine is at operating temp. any help with this is greatly appreciated. thanks