Cat vomits regularly.

My 4year old cat has been throwing up over the past two months. 99% is food. ( dry food that has softened in her stomach) about 15 mins after eating. A couple of times it has been liquified food.

History: I adopted her when she was two. She was throwing up continually and was very gassy. We put her on hills Z/D, to test for food allergies and then D/D venison and was fine until two months ago. This is the food she is still eating. I tried to get her to eat Z/D again ( to check for another food allergy) but she refuses to eat it. I still have the bag, so if you think I should really force this, I can. When I first tried I was leaving the country for 10 days and wanted to make sure she was eating before I left.

I took her to the vet and basic blood work and a wellness test came back normal. She then had an ultrasound and it came back normal

My vet did a trial of prednisolone for one week and she did not vomit.( I can't remember the dose, it was a pill)
He then switched her to Budesonide 1mg/ml (dose is 0.5ml twice a day. ) when the ultrasound came back normal. She is taking this now. She now will go 5-6 days without vomiting, but will still vomit 5-10 minutes after eating Periodically. She also vomited diluted food liquid once while on this medication.

My vet is not sure what is wrong. I want to avoid exploratory, biopsy surgery. I had a previous cat that had IBS determined by biopsy of organ surgery and she almost didn't make it through the recovery.

Anything you can suggest will be appreciated. She shows no signs of problems, except vomiting. I have another cat (male) that she is dominate to.

I look forward to your advice.
Thank shannon