My cat has an ongoing thing

Once every so often my cat will start panting and be lethargic, if he tries to stand hes all wobbly and has poor balance. He's salivating at the mouth to.

We took him to the vet at least twice and by the time we got there he had stopped.

Blood work is very expensive so its likely only a last resort option.

He usually comes back right as rain after 30 mins to an hour.

Any ideas? We're at a loss and the vets had no clue either.