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So the numbness comes and goes? always in the same locations? any other symptoms (e.g headache, ringing in ears, hearing loss)?
It sounds like a form of trigeminal neuralgia, though not the painful type. If one is young, as you are, and only part of the nerve is involved, it can be felt as numbness, tingling or some other altered sensation. But this is the nerve that supplies the region you describe. Somwhere along its path, it is being irritated or temporarily compressed, perhaps. The normal MRI would suggest it is not a problem in the brain near the origin of the nerve, fortunately. Do you get popping sensation when you chew? Do you grind your teeth at night?
I would get a CT scan of the temporal bones and temporomandibular joints. These structures do not show up well on plain films and cannot be "seen" from the outside. Since the symptoms are neuropathic, they may respond well to a nerve-stablizing medication such as gabapentin or pregabalin. I would not give up until I had done at least these two things.