2001 X5 3.0 Head light problem

[amentges]

I'm wanting to see if anyone else has ran into this problem. I have the standard xenon's on my regular bulbs and regular headlights on my brights. A few people have mentioned to me that my passenger side headlight looks like a regular non xenon one and its very dim, yet when i park and look at them beside each other then look white and normal. Has anyone ever heard of an issue like this? Like the faster i go or more electricity i use a headlight could fade? I know it sounds rediculous just trying to figure out why it could be occuring and if i should replace it? Also (never replaced a headlight on the x5) is it just a bulb or the whole assembly?

Thanks