Occasionally while I'm driving (usually when I go over a small bump or ramp on the road), I hear a slight clunk under my hood and my Traction Control System and Anti-lock Brake System lights turn on and don't turn off until I shut off the car. The next time I start up my car, the lights stay off until the same thing happens again. I don't feel any difference in the performance of my car (before or after the lights go on), though it may be too subtle to tell.
What could be the problem, and what should I do to fix it? I drive a 1999 Pontiac Montana.