My Three Year old Shih-Tzu seems to be throwing up a yellow saliva like substance every other day.

We keep him on a good lean diet, with limited treats. Anyone have an idea what may be the cause of this?

Likely the vet will have to figure this one out and fix it. Is there any chance he is managing to supplement his diet behind your back? Not as likely for him as my 3 month old Lab. I just caught myself leaving the fence to garden open and her in there eating corn stalks. Even foraging usually shows more evidence than you describe.

Yellow froth like is most commonly acid in tummy. What has he been eatting? Does he currently take any medications? And is/has there been any stress?