What Causes Excessive Talking In Children?

I am a therapeutic foster parent with a 5 year old that never stops talking ... and is always asking questions for verbal affirmations. After a year, I am not having any success, at all, with exercises of training her in timed quiet sessions, handing down negative consequences or loss of privileges. It stresses those in the home and kindergarten teachers are dealing with it too, with nothing to offer. I am entertaining the thoughts that this seems to be a severe condition that she cannot self-control. I am working so hard to keep her med-free. Is there a solution ? Is there a medical term to describe this? Is there a history of evidence that has shown that this can go away?????