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I'd try swapping the parts (ballast, igniter, bulb) one by one from the RHS to the LHS headlight to try to isolate the problem. I'm guessing the ballast or igniter is going bad and shuts off when voltage drops below a certain point. It probably happens only after 20 mins of driving because whatever part is malfunctioning starts malfunctioning only after it gets warm.
If swapping the parts from R to L does not change anything, then it could be the voltage going to the RHS low beam is dropping below the normal range. Could be something as simple as dirty contacts on the conector or fuse, broken/frayed wiring.
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