BMW: 1989 BMW 320i battery and electrical, bmw 320i, bmw 320is

Question
I bought the car (89 BMW 320is manual) and when I got it the car was havign a problem charging the battery. I took it to a mechanic and he fixed the alternator (he said there was a short) and I drove it fine for the past 6 months.

3 days ago, I was driving the car and it started shaking and sputtering and would shut off if I took my foot off the gas or came to a stop.  After about 45 minutes the problem went away.  I parked my car and left it overnight.  When I went to drive it yesterday, the car was completely dead.  Even the portable battery charger / jumper I have (300 amps) is not able to resuscitate the car.  Any ideas or help would be greatly appreciated.

Answer
Jeff,
 The problem is either the battery or the alternator.  The mechanic may have "fixed" the short, but if you have an old alternator it's still going to wear out.  So if the mechanic did not replace the alternator, but only fixed a short in the wiring... it's probably time to replace it.  
 Your Battery could be at fault as well, because they usually only last 3-5 years for most people.  I would take the car to a local autoparts store and have them test the Alternator (usually free) to see if its charging the battery.  If it's not... then there's your problem.

 Hope this helps,
 Josh