bradland

Passenger headlight (HID) turns on when switched but goes off when I start to drive. I can pull up to a store front and see it's off, turn the lights off and back on again, it comes back on but ultimately keeps going out when driving. Normally my dash will tell me if a bulb has failed but nothing for this. Is this just the bulb dying or does the ballast need replaced? I know I can switch headlight bulbs and narrow this down but I thought I'd post this first. I tried searching previous threads but opinions are about 50/50. Looking for a more definitive answer before I buy the wrong part...