Check the back of the bulb where the wires go in. Sounds like it is loose. Was going to say you have a bad relay, but if only one side is going out, I would suspect a loose wire.......
Does your Q have the regular yellowish halogen bulbs or does it have the HID Xenon bulbs? If halogen id say try a new bulb but if its the factory HIDs then there are many other components involved like ballasts. I had the replace one of the HID bulbs on my Q and the bulb alone was $190 and because the passenger side was still fine, the colors of light were off slightely and that bothered me big time hahaha.
By the way its you have the whitish blue HID bulbs then it sounds like the ballast could be on its way out. The ballast is required to boost the voltage.
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