I ran into a similar problem with charging system, which turned out to be caused by a burned out grounding strap from alternator to the engine block. The strap was held together by only a few strands of wire, which apparently was sufficient to allow proper readings under no load conditions. I inadvertantly discovered this when I dropped a nut in the process of reinstalling the alternator, which I had taken to the shop to have checked out. I had to remove the belly pan to retrieve the nut and saw the condition of the strap when I looked up. Problems went away as soon as I replaced the strap.