96 4.3L hard to start and back fires

I have a 96 1500 with 4.3L few moths a go I was driving it and it started to buck and surge. Ever since than it is hard to start in the morning and when it finally does start it runs crappy for about 45min. of driving than all problems go away till next day. Replaced thermostat and the eng. runs at 190F . Here is the funny part when its cold out side 50F or less the truck starts and runs fine>??Parts I replaced:Fuel pump , spark plugs and wires,rotor and dist cap.fuel filt twice, oxy,sensors and iac valve.No fault codes.Iam ase certified mech. cant figure it out if you do you'll be my hero.