GM Repair Problems? Ask GM Repair Expert Now.
Tony, I am Cam.
To start with, do you have ANYTHING aftermarket installed at all?
Ok, have you tried swapping out the relays for the powertrain with others of the same numbers in the underhood fuse block? Its possible that the contacts are sticking and causing this.
If no, please do so. Also inspect for signs of corrosion in the underhood fuse block as well.
Also check the ground wires. This sounds like power is being diverted from some circuits to others. There are grounds down on the engine block, on the frame under the drivers door and on the cylinder heads. Make sure all those are clean and tight.