My dog is a jack russel cross, 2yrs old, he is lively enough but he goes of his food every 2 or 3 days, he is then sick, its not food he bring up but a white frothy liquid, then he gets his appatite back

How long has this been going on? Has he been on the same food since this started? Has he been checked by a vet?

Been going on for a couple of months, we have been changing his food just to see what he liked, vet checked him and said he couldn't find anything wrong

When you change his food do you do it gradually, or just buy new food and give it to him?

This could be allergies, this could also be an upset stomach from the changing of food. It's best to find a diet that works and stick with it. Dogs have a very hard time accepting changes in their diet, and it can cause problems. It sounds like this may be what's going on, but that's only a guess.

How long have you had the dog? Has this been going on since you got him?

If not, then I'd switch back to the food he was on before these issues started happening.

Make sure to gradually change diets. You should start by mixing the old and new food together, half and half, and then gradually reducing the old food until he's completely on the new food. This will be easier on his system, and it will also help you pinpoint if he's allergic to the new food.

Good luck. :)