The problem may be under your underhood fuse block. Rodents like to climb inside under the fuse block and chew the wires. You may have the ignition E wire shorting out. I've seen this many times, but anything is possible. Also check the ground wire that runs behind the engine on the left bank cylinder head.Its not a large ground but a 14 gauge(I think) black wire that runs in the wire harness on the top of the intake that runs towards the back the engine.
You could also remove the A/C relay to help eliminate a possible feed back coming from the A/C compressor. In a lot of cases there is a diode to the compressor that can fault.
Any circuit that you can eliminate by either unplugging the unit or removing a fuse/relay will help pinpoint a fault. If I think of anything else I'll let you know. Unfortunetly this is what I do daily at work.....drivablilty and electrical diagnosis at a GM dealer
