It only dropped to 12.3? what is residual charge before load 13.2? Maybe you have too much charge for safe levels in the ECU? I'm not sure that is possible.

Does your car have immobilizer, or / and other features? Do you have a consistent reaction when it doesn't start, even though not starting is inconsistent?, is it in "ranges" or does it restart randomly, or on the __ try? etc.