I once had a kitten that would crash like that, in cycles. Turned out to be a birth defect of the liver, something that normally changes after birth, but did not for this kitten. But again that was a very young cat, who never had the years of normalcy that your cat has had. I'm adding the story because I remember the vet saying something about the build up of toxins, that would normally be filtered out by the liver, they'd get very high in this cat, he'd crash, recover, repeat in a few days. That was years ago, at a different vet. We're at River City Veterinary now. They are amazing, knowledgeable and kind. Good luck.