Something you might look at is the MAP sensor. I have a 2000 with a 4.3 that had probems with a seal around the sensor that would seem to get worse when it was cold, but sealed up once it got warmer and more pliable.
Not saying thats what it is, but you might squirt some thick oil around it to see if it makes any difference. If you pull it out you might see what I'm talking about as far as being loose.
I don't know about a 6L but I know I had intake gaskets that went bad because of the dexcool or whatever it was called coolant from the factory.
I also had to replace the central fuel injection with a multi port kit that got rid of the poppet valves. Don't know if thats on the 6L or not.
Your computer is working or it wouldn't run period as that is what turns on the fuel injectors, as well as the spark control I believe.