Suggest Treatment For Thickening Of Saliva

For the last 18-24 months my saliva have become thick (the trouble started when thick saliva kept flooding my month, which it no longer does) But now I have bubbles forming continually in my mouth. Between the tongue and hard palette, and under my tongue. It goes on all the time and stops me getting to sleep at night. I have attended the local hospital and the consultant told me to drink 2 litres a day, not including tea and coffee. I have done that for 2 months without any improvement. He said he didn't know what the cause was and so doesn't know how to cure it. (at least he's honest). He said drinking the 2 litres a day was a long term project and to go home and try not to think about the bubbles. Impossible!