Yet another 350 tbi idle problem

[Gradleg]

My grandpa restored his 1987 4x4 350 tbi silverado for my 16th birthday and as his dying wish to see it in its orignial condition. It now has perfect black paint and really nice interior but since he passed away this truck has been problem after problem. When i fire it up it was idle smooth for about 5 seconds then idles really lumped and backfires through the exhaust and spits black smoke i have to keep my foot on the gas or else it sputters and dies after about 1 minute or 30 seconds of this it mellows out and idles perfect. This only happens when i start it. . Ive replaced the iac, both coolant sensors, and the egr. Is there some thing im missing? I recently had my ecu replaced bc some water got on it while i was driving. This started after my ecu went bad and the idle has progressivly got worse. Also my transmission keeps down shifting into 3 and only locks down some times? Do yall have any tips or know of anyone in the midlle georgia area that is good with tbi and can help me out?