Have a red rash on both sides of my mouth. Can anyone help?

I've had this for a few years now, but it's been coming and going months at a time. Daktarin (a fungal infection gel) has helped in the past but is becoming more unrespondent. I eat lots of red meat and fruit and veg to help my immune system but it still doesn't help that much. I notice that the rash comes usually after a night of heavy drooling in my sleep. It isn't sore, but unsightly with flakey skin. What is it and how can I get rid of it for good without coming back?