What Causes The Need For Constant Swallowing Of Saliva?

I get saliva swallowing symptoms just as we get when we have severe cold and we swallow some of it. But this type of symptom has been going for the past 4 to 5 months. This starts every time in the evening sometime about 4pm and keeps going till bed time. it is more after consuming food, sometimes even not without food hours. I have tried different medications with doctors always suggesting dust allergy but I am not seeing any results for such a long time of medication. I would like to know whom I can refer the case at an earlier stage rather than regret later.