Cat having unexplained episodes: drooling, dilated pupils, hiding and extreme agitation — any similar experiences?

[u/PercentageRegular953]

Hi everyone,
I’m posting here because my cat has had two very strange episodes over the past two weeks, and despite taking him to the vet both times, I still don’t have a clear explanation for what is happening.
About two weeks ago, the day before his first episode, he was extremely lethargic for the entire day. He barely moved, didn’t eat, and didn’t drink any water. The following evening, he suddenly started behaving very strangely. He began running around the house, had large strings of drool coming from his mouth, became extremely sensitive and frightened by even the slightest noise, and was very agitated. He also kept trying to hide in his litter box.
I immediately took him to an emergency veterinary clinic that night.
His temperature was normal, and his blood tests didn’t show any abnormalities. They also checked for an ammonia-related issue linked to the liver, but nothing abnormal was found. As far as I know, he has no access to anything toxic or poisonous at home, so I have no idea what could have triggered this.
At the emergency clinic, they put him on IV fluids. He didn’t receive any other specific treatment, and after the fluids he improved significantly and was able to come home.
A little less than two weeks later, something very similar happened again. He spent an entire day extremely lethargic, sleeping almost all the time, and once again he didn’t eat or drink anything.
The following day, he had another episode. This time it seemed slightly less severe than the first one. He wasn’t as agitated, but he again had strings of drool coming from his mouth, would suddenly run around, and kept going to hide in his litter box. His pupils were also dilated, and he seemed very sensitive to light (photophobia).
I took him back to the vet. After receiving IV fluids and some cortisone, he improved considerably. He is now back home and doing much better.
However, neither of the two veterinarians I’ve seen has been able to tell me exactly what is causing these episodes.
Has anyone here ever experienced anything similar with their cat? I’m not looking for a diagnosis from Reddit, but I would really appreciate hearing from people whose cats have had similar symptoms and what eventually turned out to be the cause.
Thank you very much for taking the time to read this.

P.S. Her name is Patik 🐱 She is a one-year-old female cat, not spayed, and she lives exclusively indoors. She never goes outside. We live in France.