its one of those great mysteries!! I used to own a camaro v6 5 speed that if I ran it for a while, it would not restart for at least 20 minutes. Something with it was just not working. I tried the ignition, straight wired the fuel pump, new plugs and wires, EVERYTHING!! except for the stuff like crank sensor, new distributor, all the sensors that you could replace. I did all the old school stuff, and now it takes all the new sensor based diagnosis to get it figured out. If you are trying to sell it before winter, it may be worth having the dealer take a look at. I know you dont want to consider that option, but i think we are about out of suggestions except for replacing the ecu with one that works, and if that doesnt work, the replace the harness in case something got in there and chewed through something.