Mouth Sores

My mother is 60 years old and about 2 years ago she had some sores in her mouth.  The doctor thought that it was thrush, however nothing they gave her helped.  They sent her to an ENT, a Dentist and a Maxillofacial Surgeon and every treatment that they gave her did nothing for the sores.  She is unable to eat hot or spicy foods and they are very sore.  Her mouth even gets so dry that she can not open it until she pours some water in to get it moistened.  We have seen on the interenet that there are others who seem to have the same thing and have the same problem finding a doctor that knows what it is and what can help.  Some people on the net have said that it happens around the time of menopause.  My mother has had this for 2 years and we know of another lady in the area that has had the same problem for the past 10 years.  Does anyone know anything about this that can help??