HELP guinea pig has uncontrollable head movements!?

When I adopted her from pet-smart, I already had 3 guinea pigs(living in a huge c&c cage). She already had the uncontrollable head movements when pets mart got her, so they had to adopt her out for free. I had room, and they weren't taking good care of her, so I took her home with me. She eats and drinks fine, and is a very sweet pig! She does this thing with her head, like jerks it around when she walks, and she's kinda tumbly. It's not scurvy or an ear infection, because I have already treated her for that and nothing changed. I looked up about strokes in guinea pigs, and there wasn't that much info. I'm thinking that she may have had a stroke, or maybe brain damage from birth. ( I know where pet-smart gets there "pets" from and trust me, it's not a good place) Could this be a cause of inbreeding or injury? I just want an answer from some one who knows a lot about guinea pigs, please no stupid answers! I just want to figure out if it is a treatable condition, and where it came from. And please don't tell me to see a vet, because I already have. Thank you and 10 pts. to the best answer! :)