When I'm parked I can press on my brake pedal and the brake lights come on. When I'm driving, if I don't press hard on my brake pedal, my brake lights don't seem to come on. If I press hard on my brake pedal it makes my car jerk when it stops. When I press lightly on my brake predal, my car stops smoothly but my brake lights don't always come on. I just got pulled over for my brake lights being out, but when I got home, my brake lights worked just fine. I went out again later that night and my brake lights had stopped working, so we tested them in a parking lot and that is how I know they only work when I press hard on them.
The brake light switch is out of adjustment. Check the bracket it's snapped into to see if it can be moved away from the brake pedal. If you find the cruise control will kick out with a slight tap on the brake pedal, the adjustment is okay but the contacts for the brake lights may be arced away. The switch would need to be replaced for that.