Cold start idle

Truck is a 2001, 2500 with the 6.0 gas motor with 128K miles. If I try to start it after it has been sitting for 8+ hrs, it won't idle till it warms up a little. Normal idle is around 800 rpm. When I start it, it goes up to 1200 or so then drops down to nothing and dies unless I have my foor on the pedal reving it slightly. It does not do this when it is warm (+30 degress) outside. Where I live is rarly above that in the winter mornings. I took truck to a mechanic a couple monthes ago, who told me that it needed the air box serviced (whatever that is). I said OK and a 100 bucks or so later, the problem is still there. I took it back and was told the injectors need flushed. I said OK (128K, I'm sure it was more than due). The problem still persists. I took it to the dealer I bought it from. They were able to duplicate the problem but they can't figure it out either. They suggested the gas and the winter blend thats added to it. I switched gas and the problem seems to be a little better, but it is still there. I went through Chevys service bulletins and did not see any help there.
Can anyone help me figure out what is wrong with my favorite truck?

Dan