94 gmc 454 hesitation backfiring

Hope someone can help me I have 1994 gmc 454 with about 300000km. my problem is that when I first start the truck in the morning it idles up and down about 800rpm to 1300rpm then dies. Once warned up the truck idles like new, but surges at low speed and has a bad hesitation around 3200rpm. once it shifts it backfires once out of the intake. I replaced complete distributor rebuit tbi,new fuel pump with 15psi,check sensors all working within specs,new wires plug blocked off egr checked vacuum leaks none that I can find. Any ideas would be very helpful.