Charging/electrical issue. Any ideas?

Well, it looks like I've been bitten by the electrical demons. I'll explain what's been happening and then maybe someone here can tell me where to start.

I've been letting my younger brother drive the car lately since he's about ready to graduate high school. Just recently he gave the car a tuneup which included a new battery. Everything was fine until about the start of this week.

He would notice that while driving the car above 2500 rpms, the volt meter would peak just around the "L" in normal and the battery light would come on. At idle, the gauge would hang down around the "N" and "O" in normal. I didn't think too much of this, because my car has underdrive pulleys.

Then, in the next couple days the car was hard starting, and then eventually the battery was basically dead. We push start the car and drive it to a friend of mine's house who is a mechanic for the Army. He sticks a battery charger on the car and replaced the positive battery terminal which needed to be done anyways. He used a voltmeter and found no abnormal draw while the car was off.

He did a few other things and concluded that the battery was probably faulty, as the alternator was putting out power in spec even though the idle was a little low due to the underdrive pulley. We replaced the battery since it was under warranty and the car ran fine the rest of the day yesterday and most of today.

Then, while my brother was trying to leave his girlfriend's house today, the car was completely dead. He pushed started it and started to drive it home. As he was driving, the volt meter kept getting lower and lower, then the tach dropped to 0, as did the speedometer. He revved the car to about 4000rpms and the gauges came back on until the rpms fell again. After a few blocks, even revving the engine would not bring the gauges back.

Then about a block from the house, the car started running rough and eventually died in the intersection. A cop helped him move the car to the side of the road and I walked down there to meet him. We let it sit for awhile, and then we pushed started it again and got it home.

It seems obvious to me that there is some kind of electrical problem that is draining the battery while the car is on and the alternator can't keep up.

We've got the car on the battery charger now and will take it back to my friend later on, but in the meantime I was wondering if anyone might have had this problem before and could point me in the right direction.