Thanks for your reply. We've been trying not to worry, but when he actually falls off a chair, it gets really upsetting! That just happened last night while I was watching TV. He was in another chair not very far from me and suddenly without any warning he just jerked really violently and went tumbling onto the floor upside down. When he jumped up, he hit his head against the underside of the chair, so he ran across the floor looking all dazed with his tail puffed up really big, obviously scared out of his wits. I ran over and petted him and he calmed down pretty quickly, but it still scared the wits out of me too!
He's not taking any medications at all, and hardly ever has. He's always been one of the healthiest cats I've ever known. He had a really scary respiratory infection last year that caused him to breathe with his mouth open (I'd never seen that in any cat ever before), and that was the only time we've ever had to take him to vet for any kind of illness at all. He's also the largest, strongest cat I've ever seen in my life too, actually bigger than most small dogs. He's not really fat, just very large and muscular, about 23 pounds. The vet said he looked like he might be part Bengal because of his stripes, but he's the only tabby from a mixed litter where all the others are either dark gray or calico. We always joke about how big he is, calling him "the gentle giant" because despite his size and strength, he's always been such a fraidy cat! He's absolutely terrified of the doorbell and any strangers who come walking into the house. That always sends him running to hide underneath something down in the basement. I also joke whenever we trim his claws that his feet are so big, it's almost like handling a lion's paws!
In any case, he seems perfectly fine, as long as he's not asleep. We live pretty far from a vet and money is really tight these days, so we've been putting off taking him in, especially since there's nothing visibly wrong that they'd be able to see. He always eats, poops, purrs and plays just like always, as long as he's awake. I've had what they call "night terrors" myself a few times in my life, so I'm wondering if it's something that's purely psychological. Like I said, he's always been a pretty nervous and high-strung cat, but there's nothing more stressful than ever going on right now.
He's about six years old, but these violent spasms have only started happening over the last few weeks. At first, it seemed to coincide with when we had changed to using a different kind of cat litter, so we thought it might be coming from him inhaling some kind of chemicals in the dust from that. We switched back to the same kind we'd always used and he seemed okay for about a week, but then he suddenly started jerking around again last night, so I guess it wasn't from that after all. We have one other cat and she seems perfectly fine. She's about three years older than him and still plays like a kitten!
Anyway, thanks again for your reply. If anyone else has any ideas or suggestions, please let me know.