What Causes Confusion And Difficulty In Speaking While Suffering From Dementia?

My 88 year old mother has been diagnosed with Dementia last year. The family has seen noticeable progression this year. But we are facing something we are not finding an explanation for... she has days of clear reasoning, talks about family, remember most names, asks about things she remembers from recent happens, seems happy. Then the next day she is terribly confusion, has difficulty speaking, difficult to understand, rambles, repeats the some information over and over and that s all she can tell you and acts and says she is very depressed. It like a light switch turning on and off. You never know when you talk to her what she will be like. It s really strange to us and scary. Doctor is now seeing these changes on a almost daily basis know and she just says it probably just the dementia. We believe something more is going on. Any ideas, tests. We could appreciate any help. Sincerely,