What Causes Tickling Sensation In The Throat And Cough?

I have been having a somewhat persistent cough for a few weeks now (started when I got over a cold). It seems to be worse at night and when I am not sitting straight up. Sitting in a relaxed position on the couch, or lying down makes it much worse. It feels like a tickle in my throat, and coughing or clearing my throat alleviates the problem temporarily. About 3 times per week when I wake up, I will cough and cough up a white stringy rope like substance. It looks almost like string cheese or toothpaste. It has varied in length, but is usually around 2 inches in length. I feel fine after that, and rarely cough or clear my throat during the day. But it will begin again in the evening as I become less inactive. Do you know what this is???