For the last couple of days, I have noticed my "brake light" indicator coming off and on in my dash. Experience with this tells me that one of the light bulbs is burnt out and a bulb needs to be replaced. However, this particular issue is odd in the fact that it doesn't always turn on and it only happens after the car has been running for a while. My previous experience with this tells me that as soon as you press the brake pedal on the car that the indicator light will come on should one of the brake light bulbs be burnt out. Since this isn't the case and all my brake lights appear to be working, what could be the issue?