Have this problem a lot on the HID work lights on the mining gear at work. It is a sign they are on their way out. Will work for a while till they get to warm and will then die. Flick em off for 30 seconds and they will come back. For a while anyway. Sorry only fix is to replace them.

If you have bought them new recently chase him to replace it as it is faulty.

__________________
Stoner & Webber 2011 World Champions