The best Idea I have involves beer and a old buddy of mine who is a Chevy expert. I too can only guess on this system. one of the things that comes to mind is your ECM maybe having issues with the open loop function. its got 3 minutes to get the engine warm and after that it goes to closed loop to meet emissions. I am wondering if you have a bad injector or TBI Gasket is sucking air on one side real bad making a lean condition until the computor makes it to closed loop. its just odd. I did have one years ago that had a cracked intake and was sucking air under the tbi through the crack that went from one side to the other..