This might sound crazy and it could be totally unrelated but... before I stopped eating gluten, I got sinus infections a LOT. About twice a year, and every time, it was guaranteed to turn into bronchitis. It would knock me out for a minimum of a full month. Since I stopped eating gluten (almost three years ago now I think?), I have only been that sick one time. Otherwise I will have like a day, maybe two, where I am not feeling 100%, I've got a sore throat or my head feels really foggy... and then bam, the next day I'm fine.
It seems weird to be related but I feel like it is, because that's the only thing that changed between when I was sick a lot and not being sick a lot. My co-workers here get sick ALL THE TIME and I never catch anything from them. The whole no-more-gluten-because-your-body-hates-gluten thing seems to have my immune system where it should be and it steamrollers over most illnesses.
I also think my immune system improved somewhat when I was working out regularly, but definitely not with the night and day change from when I stopped eating gluten. I know you mentioned trying a gluten free diet before, how did that go? Do you think it might be worth trying again?
Otherwise I agree with checking to see if there's anything else going on. I had an MRI done on my brain a few years ago for something unrelated and the doctor said I have chronic inflammation of the sinuses, which is probably what caused my problems. If my sinuses were already inflamed just as a normal state of being, any minor irritation was quick to cause major issues. (I'm not normally one of those who's on board the whole GLUTEN CAUSES INFLAMMATION bus, and I haven't had another MRI since to see if it has changed since I'm no mo' gluten, but that is another possibility.) It didn't really give me any answers to STOP from getting sick, but it at least shed some light on what was going on. He mentioned that if it continued, I might be a candidate for surgery one day, I don't even know what that meant because as soon as I hear surgery I'm practically running for the door, but it sounded like there was something they could do to reduce the inflammation.
For the immediate, the only thing I could take that would help even slightly when I was sick was Sudafed, the real stuff you have to get from the pharmacy, not the stuff you can pick up in the aisle. It wasn't very easy on my stomach but it was the only thing that could make me feel human again. Sinus infections are a serious PITA, I'm sorry!
ETA: I forgot - I use this for when the sore throats are killer, too. It doesn't resolve the issue but it did make it stop hurting for a few hours at least so I could get at least a bit of sleep (my sore throats would be so bad it would keep me awake at night.)
http://www.vitaminshoppe.com/p/quantum-herbal-sore-throat-spray-4-fl-oz-spray/qt-1043
It tastes like ass, no joke. SO disgusting. But it works.