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I agree that the voltage regulator is a common culprit, but don't forget about the alternator brushes either - they wear out quickly too and can lose contact.
I had exactly this problem on my old car - the voltage was jumping all over the place, but the regulator turned out to be fine. The easiest thing is to first measure the voltage on the battery while the engine is running (it should be 13.5-14.5 V), and then based on the results decide what needs replacing.
If the generator won't hold a charge, first thing you should check is the voltage regulator - that's usually the culprit. The simplest way to do it: with the engine running, check the voltmeter reading at idle - if the voltage doesn't go above 13.5V, then the problem is definitely in the regulator.
The generator itself fails less often, and you usually notice it by strange noises or a burning smell. I recommend replacing the voltage regulator first, it's cheaper and easier.
Here's a simple test: start the car and turn the headlights on full, then see if they get brighter or dimmer. If they get brighter - the alternator's working, so it's definitely the regulator or brushes. If the headlights stay the same brightness or even dim when the engine's running, the problem's in the alternator itself (probably lost output). Before you take it apart, you can also check the battery terminal connections - corroded contacts often look like a charging problem, but they clean up in five minutes.
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