When I ran HID's on my silverado a few years back they would start to flicker more with age. I was told a few different reasons this could be. Check your grounds. Don't use HIDs with the auto on feature for the headlights. Make sure the ballast is not moving around.
I did all of this with my HIDs with no luck. A little after that one side went out completely. No warrenty so I ordered another set of HIDs only to have those burn out after a year. No more HIDs for me until they last a little longer. Hope your experience is better.