98' Charging issue

Volt gauge is acting odd. Start up vehicle. Volts are +13. Upon driving around awhile, when I come up to a stop sign and apply the brakes, the volt needle will plunge rapidly under 13 volts. Now this is in the day time with no accessories on. I just sat in the driveway at idle and watched the needle move around like pulsing. I can shut off the ignition and restart the car and everything goes back to normal for awhile. Then it starts acting up again.

Have the alt out put checked. Make sure all the battery cables are clean and tight.