My Yorkshire Terrier Medium Sized, Has Had On And Off

My Yorkshire terrier medium sized, has had on and off diarrhea and vomiting over last 6 wks. Seems to be sick every Mon Tue wed and then picks up again until the start of the nx week?? 2 vet visits over first 3wks with antibiotics and medicated canned dog food and he's still not right. I've replaced pedigree dry food for organic salmon wheat free dry feed. He's good today but could it be anxiety, ulcer, or stress when we're away a lot at the end of the week and weekends,

Hope you can help

Many thanks

Sue