sounds like mine!!
i talked to the guy who is working on my truck yesterday,he said when it dies it looses all ability to get any codes from the computer,there is one of the wires that provide power to a part of the computer that acts as memory,the way he explained it was like your radio that remembers your preset stations etc,sometimes it will start and drive and not die,next time it wont start,then later it starts right up,i suggested that he run jumper wires to both the power and the grounds,he said he didnt want to jerry rig it,to which i said its not a new cadillac,just make it run!!