My mother has moments in time where she will be talking and then just can't speak for 5-15 minutes. ct and mri show nothing. what else could it be?

Aphasia is the name: for the symptoms you describe. The brain knows what it wants to say but doesn't connect w/the voice. I don't know if your mom has this and encourage you to get a second opinion. Also, a neuropsychologist can do extensive testing and offer a diagnosis and ways to deal w/this. Peace and good health.

ER now: She will need further neurological evaluation as well as bilateral carotid ultrasounds: never ignore these symptoms and return to the ER for further evaluation and observation-TODAY!