All alternator/generator output is a function of engine rpms. The faster the alt/gen spins, the more voltage/current it will generate. When rpms begin to fall below 900/1000 the output performance really falls off quickly. You don't say what your rpms are falling to. It could be that your engine idle is too low. This is why when you jump start another car, you raise the rpms on the engine to about 1500 because it brings the voltage up and thus the current being provided to the dead battery. Your Sub is also old enough to where the battery is a possible suspect, but if you're starting fine, it's probably ok.