My wifes '03 has started vibrating or shaking in the front end when she brakes. This only happens occassionally, not every time she brakes. I just put new rotors/brakes on it at the end of last year and it was also aligned last year. Someone mentioned the rotors may be bad/warped but I would think that would shake anytime she brakes. What else should I be looking for? Any help would be appreciated.

Thanks,
Jason