I had a problem like that on an old Ford 7.3 that ended up being a loose injector harness connection. I don't know how the Duramax works but it might be worth looking into.
I just glanced through this but you might take a look
http://www.duramaxforum.com/forum/911-problems-forum/2600-2005-gmc-duramax-wont-start.html