Possible seizure?

About 8 months ago I noticed Gilbert (7m) was walking sideways and when I went to goce him a treat he fell over backwards, so I rushed him to the vet but long story they really couldn't do anything. Then tonight he did the same thing but his leg was moving back and forth and his head was shaking. Both only lasted for about 1 minute but it was terrifying. Anyone have any idea what this was and can anything be done about it. He is a few pounds overweight.