4 yr old neutered indoor shorthaired cat.  He almost never vomited til 1
month ago.

1 month ago, he vomited frequently for 2-3 straight days, took him to the
vet, they took blood tests and xrays, said he had what looked like "trapped
gas", maybe blamed on food.  After an enema, he came home and was fine until
5 days ago. Then he started vomiting again, 3-4 times per day, for 2 days.
Took him to another vet, they took blood test, "upper GI" test with barium,
and xray, said everything looked fine, no obstructions in intestines or
stomach.  Today, he vomits again 2-3 times.

During these episodes, no fever and doesn't act lethargic or "sick" in any
way, has no problem with thirst or appetite and has stable weight all his
adult life at 15.5lbs, including now.

What can cause this and be so undetectable?  Is Irritable Bowel Syndrome a
possibility?