Mrs. Puddles, I wonder if you have allergies? I had symptoms similar to yours for (this is embarrassing!) over three years. I'd get sick, see the doctor, sometimes get antibiotics, get better ... a month or two later, I'd be sick again. This kept happening. It got to the point that when I came into work I'd be coughing so loudly and horribly that the receptionist upstairs (who couldn't see me) would call down: "Hello, 2Shy!" lol Well, she didn't say "2Shy" but you catch my drift.
Doctor sent me to an ENT and I had all sorts of tests. They thought I might have asthma. Finally, the ENT sent me to an allergist even though I had never had allergies in my life. Well, let me tell you, they pricked my back with just about everything. Turns out I was allergic to bluegrass (the plant, not the music), maples (there were 3 in our yard) many, many kinds of trees. Worst, I was allergic to dust and cats--both of which I had plenty of!
The allergist explained that my symptoms came back due to the dust allergy and had to do with the life cycle of those creepy little dust mites. I started on shots (though I have discontinued those).
I also had a chronic sinus infection that would get worse and better along with the allergies. I had never completely gotten over it. It would lessen just enough for me to feel better, but then with weather or more mites, it would worsen. Hence, I'd get "sick" again.
With massive antibiotics, a steroid shot and steroids, I finally whipped the sinus infection. I am prone to them due to my allergies and the narrowness of my sinus cavities.
Anyway, if you have not considered allergies (I denied I had any for years) you might want to have them checked.
BTW, I still have cats, dust, and dogs.