Cat is swaying gently while sitting

I have an 18 month old neutered male cat. Around one month ago, I noticed he was swaying gently, was somewhat clumsy when jumping, and was acting nervous. I took him straight to the vet, but they couldn't figure out what the problem was and ended up giving him an anti-inflammatory shot and a course of antibiotics. The symptoms passed completely by the time we got him home from the vet, and everything seemed fine until today (Oct 19) when he once again began showing the same symptoms. What could be causing his problems?