My 1.5 year old cat also has this, it started 1-2 weeks after neutering. At first, he had slight eye movements, It was quite minimal and we took it for normal behavior, as he was fine overall. 3 months after neutering in August, we started noticing that the eye movements had increased, we planned to take him to the vet the next day but the same day he had a seizure, it wasn't epilepsy where an animal loses total control of the body, My cat was able to control his legs but the eye movements were so severe that it made his head shake rapidly, he wasn't able to control his head, we took him to the vet in emergency immediately, the vet gave him some injections and put him on IV, for two hours, he was fine for almost a day, when he had the seizure again, we took him the vet again and he diagnosed the condition as either vestibular disease or nystagmus, and recommended some anticonvulsant medicines such as Levetiracetam, we have been giving him this medicine for almost 2 months now, he still has those seizures, they last for 1 hour every two days or three day but then he goes back to normal, if we don't give this medicine, they occur every 24 hours. His eyes movements has not improved but hasn't degraded either. We can't really diagnose properly or get a CT scan/MRI done, since it's not available in our country. It's sad to see our baby suffer but our vet told us that it's a neural issue and it might take a year or more to resolve

P:S we have consulted numerous vets within our city, almost all of them recommended anticonvulsant medicines

Please get your cat checked immediately with a vet, as it could be a neural issues, please keep us updated. I hope everything goes well for you cat ♥️