2003 GMC Sierra/Anti-theft deterrent light

I've got a 2003 GMC Sierra, 2500 HD-WT which has a problem with the anti-theft light. The light comes on intermittently while driving. Have been back to GMC dealership 6 times. They cancel codes in PCM & BCM, but light still comes on. This has been going on for a month now, have been stranded 2 times because of no start. Fuel pump will not run, because vehicle thinks it's being stolen. Any input?

Thanks to GMMerlin for your input. You think that the dealership would know what to do by now. They seem to have done what you suggested, but still does not take care of the problem.