the reason it is bright when your lights are off is because the factory figured when the lights are off the sun is up and to see the time it has to be brighter so you can see it with all the ambient light around you. and it dims when you turn the lights on so it wont blind you while driving at night. as far as it brighting and dimind when you hit a bump is prob due to a loose connection in the conductors to the LCD inside the clock. you may be able to repair it but it will involve disasembling the clock and resoldering the conductor strips on the printed circuit board ... not worth the hassle not to mention how delicate a procedure it would be ....... when and if it goes out replace it with one from a junk yard. David