I have an HID but the light will drop low when in stable staate..can you help me about it?

im using an 4700uf 16v capacitor connected before the ballast input..or should i say parallel to the ballast input..i try to use another bulb but still the light will drop..at first it will light very bight then after an certain tym the light will drop..it like and super white halogen..please help me..