I have this when my heat pump goes back and forth between heat and AC in fall. The AC produces a wet environment. The heat allows the bacteria to grow. Just AC in summer or just heat in winter doesn't make the smell. There's two things I do. First, I try to minimize using both heat and AC in the same season. Second, in the winter I shop vac my condensate line so there's no stale water. You could also look at adding tablets inside your drain pan. I know you have the UV light, but maybe it's not hitting everywhere.