01 gmc shutting off while driving

I have a 01 sierra 3500 with the 8.1. a few months ago it started shutting off while I would be driving down the road. Took it to the dealership and they said it was missing a stall code in the PCM. They installed the code and it worked fine for about 3 weeks. It had gotten to the point were it was shutting off 3 to 4 times a trip and the check engine light finally came on. Onstar said that the PCM was reading an ignition problem, but when I got it to the dealership they said that it was not showing a code. They drove it for 70 or 80 miles and it never cut out on them so they don't know what the problem is. If anybody out there has had this problem before or knows the cause the info would be much appreciated.