Don't be too sure that diet isn't a problem - some foods suit one dog better than another. Certainly diet seems to cause this problem, and in the past when I've had the same prob, a change of food solved it.
Make sure you're not overfeeding, that also could aggravate the problem, as it can be caused, I understand by breakdown products of the food. Bad doggy smells are the same.

Last year, i noticed my girl had really bad breath, so i checked her teeth and found she had broken 1, and it was loose, i was horrified ! Took her to the vet and had it removed, cost £160 and had her teeth cleaned for that much as well, but cos it had been recommended by the vet to be removed, i could claim on my insurance.

take advantage of it being pet smile month, see if any of ur local vets are participating and get an appointment for a complimentry mouth,teeth and gum ck. If it is an oral prob they should spot it and advise u on the best course of action aswell as give u some freebies!