Himalayan cat with neurological symptoms?? Head tilt, rapid eye and head movement.

I've come to help out a 3 year old unspayed purebred Himalayan female. She seems to have some neurological issue.

Her symptoms are continuous head movement mostly side to side and her eyes also move slightly side to side. She ***** her head to the left. I did notice she somtimes circles to the left . She is not unsteady, can jump and navigate well. She is eating, drinking and using the litter. She is content..plays with toys. She was take to the vet who couldn't find anything physically wrong by a physical examination. She saw nothing in her eyes that concerned her and treated her for a possible ear infection even though nothing was indicative of that.Her former owner says she has had these symptoms

since a kitten and that's there's nothing wrong with her. (her word means nothing..she pretty much abandoned that cat for over 3 weeks)

Does anyone have any idea what the problem might be? I don't want to spend alot of money investigating if she seems healthy otherwise but I'm trying to find  her a home and want some answers for people considering her. I want to figure out how severe her medical needs may be.

She does squint the left eye more so and it seems watery at times.
Thank you!!

Is the third eyelid up in one of the eyes? Are the pupils the same size? She doesn't lose her balance? Is the eye movement rhythmic and side to side?

Did you get a diagnosis for the symptoms? We just received a Siamese adult male in to our rescue with the near identical symptoms.

Maybe a physical defect in her ear or some past trauma? Can she eat, drink, and use the litter box alright?