Have the charging system professionally load-tested. In particular, if all you can get is one third of the generator's rated current, and "ripple" voltage is very high, the generator has a bad diode. That is caused by the generator's design, but the battery helps to overcome that. To reduce the high number of repeat generator failures on '87 and newer GM products, replace the battery at the same time as you replace the generator unless it is less than about two years old.
By now you've figured out throwing parts at a problem is the most expensive and least effective way to diagnose a problem. On GM vehicles, the generators develop huge voltage spikes that can damage the internal diodes, the voltage regulator, and can interfere with computer sensor signals. The generator is the most common cause of intermittent, elusive engine running problems that defy diagnosis. Start with a fully-charged battery, disconnect the small plug from the rear / side of the generator, then drive the car. If the problem has cleared up, replace the generator and battery.
Monday, March 23rd, 2015 AT 8:26 PM