brake lights problems
 

I have a BMW x5 4.6is 2003. having problems with the brake light, when i press on the brake pedals, the light goes on but don't stay On, after a few seconds they go out, will I'm still on the brake pedals... And while I'm driven they come on when every they like, or not at all...I have check the plastic casing which hold the bulb it seem OK no rust on it....need help