Do you have any anything special installed, such as an amp or any engine mods?
Where did you buy your alternator and what brand is it?
Have you had it's output tested with a voltmeter?
Alternators for domestic cars are not exactly the most well-built components. I've known many people to buy rebuilt or new alternators that couldn't provide stable output, and they had to be exchanged, sometimes multiple times.
Does anything other than stopping seem to set it off? Try turning everything off in the car to see if it still occurs.
It may be that your cooling fans engage and draw too much current from the system once you're stopped. I'd take a look at them for sure. Try idling and turning on your lights, as well as either your AC or Defrost, since it engages the AC pump and fans as well, to ensure the AC doesn't lock up during the parts of the year when it's not needed.
If your voltage drops when the fans engage, I would have them and their wiring inspected.
Thursday, May 17th, 2007 AT 1:16 AM