Glitchy Brake Lights

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.