What Causes Bleeding In The Saliva?

i spit blood very regularly. been to multiple docs and no one can figure it out. Caridologist, neurologist, endocardologist, pulmonologist, 3 ENT s,dentist, primary doc. have had multiple ct s, mri s, upperendoscopies. blood seems to just appear, never cough it up, mouth gets very dry. i will split alot in saliva and then it will be gone, not consistent. my blood pressure and anxiety are all over the place from not knowing, but it is a material amount of blood on occassion. Sometimes bright red sometimes brown. Any idea? could it be high blood pressure causing bleeding? anything else, desperate for answers. Has been going on everyday for 18months