I'd also try switching to a fish-based limited-ingredient diet (Fish & Sweet Potato--there are several brand that are available like Natural Balance, Taste of the Wild, Nature's Domain, and several more). The fish has agreed with dogs I've had that are sensitive. I avoid chicken with sensitive dogs, as it seems a culprit for some of them.
Since the infection is recurring, you might look into a treatment for ears called Oti-Pak, available from your vet. It is SO much better than dribbling antibiotic ointment in the ear and rubbing it around. The vet packs the ear with "goop"
(a medicated wax, I think). It dries and then stays in there at least a week and slowly desolves, treating the ear the whole while. You don't have to do anything with it, other than very gently cleaning any oozing that may come out as it breaks down. Our rescue's vet uses it on the dogs who have the grossest, nastiest ear infections, and it works. It's so
much easier than struggling with the ointment with a dog whose ears hurt. We've never had to re-treat ears on a dog with Oti-Pak, and we've seen some BAD ears. (They don't seem to hear well while it's in there though -- I think it pretty well plugs up the ear until it dissolves.)
Here's a video that explains it: