I would be wondering if there is a noisey/bad connection to the canbus network as it does NOT take much to cause weird things to happen
but starting more "basic" I would be checking the ground side wiring to both the engine and cab/frame or better yet add a ground strap from the BATT NEG to the engine AND cab
Your probably on to something here. The trouble is that with today's complexity the average dealership isn't going to know or care to try and properly troubleshoot this. If it's not an outright trouble code or they don't see it happen on the test drive they will continue to tell the customer that nothing is found wrong. You may need to try several Chevy dealers and really complain about this before it is taken seriously and properly looked into.