What Causes Sores In The Mouth?

My Mom is 82, and for years has had sores in her mouth that leak a fluid or pus, and this is also very painful to her. She has been to numerous drs. and specialists but no one can help her cure this. She does get a numbing medicine from her primary care dr. that helps take the edge off the pain a little, but does not cure it....She has had this condition for at least 10 years. She has false teeth with a partial plate, but still has her lower four teeth in the bottom front. She is thinking about having her teeth taken out, thinking that it may help the mouth condition....Any input at all on this would be much appreciated....