G01 X3 Headlight Malfunction

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I'm getting an issue when driving at night where the headlights go into 'driving on a dark country road with no cars about' mode almost every time. Ie even when driving on busy roads, the headlights are almost on full beam. Embarrassing for me, annoying for other drivers. They start off in the correct position, then move up after about a minute of driving. I know a can manually adjust them down, but this feels like more of a sensor issue, or something like that. Anyone got any ideas? I spoke to a local specialist, they'd never heard of this happening. I know there's a sensor behind the rear view mirror, which I've made sure is clean.