I had a nagging cough unrelted to congestion for several months recently too. There are many things that can cause such a thing, but in my case, it was acid coming up from my stomach irritating my esophagus. I fixed it with aloe vera, bromelian and a few other stomach soothers....
This came as a shock to me, as I had gone to a doctor and he didn't have the slightest idea what it could be. It was an herbalist here on CZ who suggested acid reflux. I had no idea my stomach could cause a cough...I never would have guessed in 1000 years.