Sounds kind of like the ABS acting like ABS brakes do. Nothing to worry about, IMO. If it gets worse or shows other symptoms, check with a mechanic. If you don't have ABS, I'm not sure what it'd be. There's only so many things to go wrong with the brakes on a car, you know? ABS brakes, when cold or when wet, have a tendency to act spaz-tastic like that though...