GMC Troubleshooting Questions? Ask a Mechanic for Answers
I have to ask a few questions just to make sure I have good base information to go off of.
To me it sounds like a ground issue. the GMCs are notorious for acting up when a ground are loose or inadequate. There are several grounds typically one at the back of the engine block going to the firewall and then the main negative cable. What you have going on is very indicative of a grounding issue. I would suggest unhooking your ground cable off your battery and ohm checking it from the end of the battery to the block and to the starter. You should have less than 1ohm resistance. You can pick up a factory replacement cable for under $75
let me know if you have any other questions, if you need help sourcing the part please let me know.