Y'know, I think we're wandering off the main symptom that should really be checked here, and that'* that the car can't seem to keep going once it'* started. Even a drain that lets the battery go dead in a couple of days should be overcome when the engine is running; the alternator should be putting out _way_ more amperage than the engine needs to stay running, including any drains like a glovebox light left on, etc., and from we're hearing, that doesn't seem to be happening here.
When you're driving the car and it starts doing its stumbling thing, what is the indicated voltage on the voltmeter?
It might also be good to have somebody install an ammeter temporarily, just to verify that the alternator is putting out proper amperage when the car is running. (Installing an ammeter can be tricky; you need to know exactly where in the circuits it can go, and use at least 10-gauge wiring for it. If you're not sure how to do it, I would recommend having a pro either advise you first, or do it himself...)
-- Andy Green