Flickering Xenon headlights

Hi all, 

Driver's side headlight has recently started to flicker whilst driving. Issue affects headlight only, no issue with dipped beam. On inspection, during the startup dance, the headlight momentarily flickers during the horizontal part of the auto adjust. Then when driving, when the headlight swings to follow the steering it flicks off, then returns, eventually it spends more time off than on. Am I looking at a bulb or ballast or auto adjust issue? Any help or advice much appreciated. 

Cheers 

Seán