Mine did this last week. I changed the brushes on my alt a few weeks ago. It had been running fine since then. Last Wednesday, I noticed the idle got low at times when I came to a stop, and the voltage dropped pretty fast. If I gave a little gas, it went back up.
I was on the interstate coming home Friday and traffic built up, so I had to stop rather quickly...damn thing died on me! I put it in park, and it started right up. Its not my alternator, as it continues to charge my battery...I never have a problem starting.
It hasn't died on me since, but the idle and voltage still drops on me. I'm going to check the MAF sensor when I get home tonight...hopefully that just needs cleaned.