Hello, I have a 1.6 YO Silky Terrier and she has had the exact same problem. I took her to two Vets and niether one had any clue what was/is wrong with her. I have found tho that I cannot give her an excess of training treats or the problem recurs. This of course makes traning her a bit more difficult.
I currently have her on Eukanuba Lamb and Rice and she seems to be doing fine. Like I said I just need to watch the treats. The vets wanted her on one of the Hills Diet foods you both mentioned, but I do not like Hills Diets ingredients. Any way I wish you luck and good health to you and you pet.
You can tell all you need to about a society from how it treats animals and beaches. -Frank DeFord