I am having a weird issue with the electrical system and no one can seem to help. As far as I can tell, it's some computer gone bad, or a bad ground. Basically, the more load I put on the system(i.e. headlights, blinkers, etc) the worse the card drives. It starts to sound like its losing compression and starts to backfire if I give it to much gas. It barely accelerates. unless I turn the blinker, or headlights off. It does not do this all the time, I would say 30% of the time at night when I have ore than just my headlights on. My A/C quit working about a year ago. I can turn the electronic thermostat up and down, but it only blows luke warm or hot. The alternator checks good, and I have re-done a few grounds. Runs pretty good during the day, when the lights are off, but I am only getting about 12 miles to the gallon, when I was getting about 18-19 befor this started to happen. Any insight or ideas would help save my sanity!

By the way, I have replaced the fuel pressure regulator, fuel pump(because it went out),plugs(wires soon to come), cap & rotor, fuel filter, and the o2 sensor. Cleaned the hec out of the throttle body and ran sea foam thru it. Shop says it gets no codes to speak of, and is getting no signal from the temp control computer(not sure what it is called, BCM maybe) Weird part is, the BCM seems to be working ok, since I can see the controlls and change them.

Shop says it gets no codes to speak of......no codes at all or they don't tell you?

Old Battery cableswill causegremlins.....Old batteries do too.
Get yourself a Service Manual for your model....not a chiltons/haynes 'repair' manual,but the GM shop/service manual.eBay Motors usually in the $20-$80 range.
're-done a few grounds'.....theMain engine/body grounds? Tight/shiney clean?