GM Repair Problems? Ask GM Repair Expert Now.
Check the connection at the main fuse box under the hood. Make sure cable is tight. You could try to wiggle wiring under the hood and try to get the engine to stall, that would narrow down the problem. I would suspect a problem around the main power cable to the fuse box since everything shuts off.
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Have you scanned for codes?
Have you checked your main grounds?
This is hard to me to help diagnose without being there, I am going to open this up to other experts. Please do not respond to this because it will lock this back on to me.