Suggest Treatment For Chronic Cough

I have had a cough for over 3 months. It is intermittent during the day, but really bad when I lay down. It always feels like I have something caught in the back of my throat. I have to prop myself up with pillows and can not lay on my back or left side. I can get some sleep on my right side, but still cough. I have to spit up every 3 or less minutes and its a clear phlem combined with what looks like dish suds. I have had a CT scan and xrays and tested for TB and those tests were negative. I have been given drugs for acid relux, anti-biotics, inhalers and allergy medicine, but it still persists. Sometimes the cough is so bad it makes me vomit. The only relief I have found is a couple sips of Hennesy at night and an ambien. I need to know what is causing it, because I am always tired and have a cough every day. I also have RA and am concerned with my weakened immune system that there is a risk of becoming worse.