2002 Vauxhall Vectra xenon headlights fault

hi my drivers side xenon headlight sometimes wont turn on so i flick on/off switch on then off a few times till it decides to work, sometime it flickers like a strobe. once its on its ok but if i turn engine off and restart happens again, same as next time i go to turn lights on. i replaced both bulbs was ok for few weeks now same again. these lights are standard on CDX models. anyone else had same problems. took it to main dealer and they cant fix as going to cost to much to find fault