Have you removed the positive battery cable rubber insulation and inspected the area between the cable and spacer on both sides. (Be sure to remove the negative cable first)! This is a very common problem on these cars with piggyback side mount cables. In fact, both cables should have the rubber insulation removed for inspection. If you see corrosion in this area, use a 25/75 combination of baking soda and water with a brass brush to thoroughly clean everything on both sides. Once you have all this clean & tight, if the problem still persist, it is time for a new ECU. Voltage spikes can easily damage the ECU. i.e. bad battery connections, or cables.
I found a permanent fix for the terminal dilemma on all my GM cars by replacing the terminals with heavy-duty soldered terminals and gold plated battery post. Never had a problem since!
See pics below:
