I would personally cull the ones with the problems unless you are able to trace it back the linage 100% and check for further deformities from the original parents. Also is it possible it could be something like an inner ear infection that is causing the imbalance problems?

We try to add in new blood at least 2-3 times per year and seems to help enough to keep them from becoming to inbreed at least to the point of undersized babies, litters or deformities.

Good luck.