1997 Buick Lesabre 136k. Electrical question.

What is your question?

I replaced the alternator and battery because it was running around 13 volts. Volts look good now -about 14. But now when I apply brakes or slow down there is a huge swing in volts; it goes back down to 13! Turn signal gets weak, etc. It goes back up to around 14 volts a few seconds later. What could be causing this drop in electrical power? Thanks!