Have you replaced any bulbs recently? If you have used the wrong size or type of bulb you can get this problem.
Could also be that one of the brake bulbs is failing, or you have a loose earth connection or wiring fault.
Try removing one brake bulb then apply brakes, if the problem has gone examine that side, if not, replace bulb and repeat on other side.
What year is the car?
vaux, August 2009
