These lesions sound like they must be a fungus of some type. I, myself, have a skin fungal infection, that is never going away, called "lichen planus", and it will get better and sometimes worse.
Dogs can get this fungus disease also. There are other fungi as well, and it could even have something to do with where you live and certain types of grasses or dirt that could be harboring this type of fungus.
I know that my infections, seem to get better when I am not in the sunlight. Have you noticed if your dog gets any worse or any better when in the sun or do these sores just appear off and on, or spontaneously?
I found the following website that possibly could have some bearing in the case of your dog, and you might want to take a look at it:
Hope you can find some answers and it is almost like you have to turn into a detective to try to figure out what is causing the sores on your dog, and if it could possibly be diet related, or environmentally related or caused by a fungus.
God bless and good luck!