My cat is 10 at this point and was checked for absolutely everything... parasites, fecal exams, etc. Multiple times during his life. And this was by a competent vet (a cat-specialist vet, actually). The best explanation, which the vet admitted is not really an actual explanation, is "sensitive stomach." Which could be caused by food allergies, which can develop and change during a cat's life.It's pretty much the same here, though we were already feeding almost exclusively wet food. Sorting out individual ingredients that caused vomiting helped a lot. Then Edwina's vomiting got much worse last year and (long story short) she had to have stomach surgery; we'll never know what caused her problem but (again, long story short) I'm leaning toward the parasite theory. Even biopsies couldn't give us a definitive answer. In any case, it's definitely worth talking with your vet about nematodes if you just can't figure out what's causing your cat to vomit.
Oh, raising dishes helps a lot in preventing regurgitation!
Also, his gut flora could be off. But I have been giving him probiotics for a long time, and they helped for a time but then stopped helping.
In short, I've been chasing this problem for almost 9 years, constantly changing how I treat it and what food I give him. Right now, he is very stable and rarely vomiting, and hasn't for the better part of a year. That is a considerable victory.
Getting him onto mostly wet food definitely was a big step in getting here.