Medical diagnostic help for my cat?

Ok my cat is a 4 year old American short hair, recently he started having random episodes of severe dizziness and smooth but uncontrolled muscle movement. During these episodes someone will usually gently hold on to him to keep him from standing up and hurting him self, this always prompts him to start purring showing he’s still aware of his surroundings and ether trying to keep his composer or is not in pain. The episodes start with him standing up and falling over, or him just lying on a bed or couch and he randomly looses balance and falls off. The episodes start sudden but they slowly subside until he’s back to normal within a minute or two, during his heart rate is very fast but stable (ie. not irregular.) and afterwards his reflexes are fine, his hearing is fine, and his eye sight is fine. My first thought was ether blood sugar crash or dehydration because he is on new low calorie diet food, we added in more of his old food so he gets some calories and it seemed to help, he had a rather bad attack today where he fell on the floor and was struggling to stay off his back, he was stumbling and rolling around, his back legs kept sticking up in the air and he couldn’t really control his front legs or most of his spinal muscles. My next thought is ether inner ear infection or he had a small stroke a little while ago and now has neural relapses. But I can’t find any evidence for neurological damage but something is going on, he is 22 pounds and last time he went to the vet they said their was a large amount of fat around his organs, which is why he is on the diet food, he has made good progress and has lost some of the fat but he is still over wait. He is rather active, he chases things and jumps up on stuff without incident. The last little bit his background, we had another cat which was his cousin and she died at 3 of congenital heart failure, as a result we had him checked and he dose have a heart murmur, which is thought to be because of the fat around his heart is preventing a valve from closing all the way. He’s sitting next to me right now and looks and is acting perfectly fine, pleas let me know what you thing and whether or not you can help me.