My friend's fater has been in and out of the hospital recently. He's in his 80's and the doctor's can't seem to find out what's wrong. One day he will be normal, knowing everyone and even playing in his band. Only days later he will look blankly at his family and not speak, even when spoken to. He also vomits when this occurs. The doctors say he didn't have a storke and that all tests so far have come back negative. Then a day or two later he's back to normal. This has happened a few times. Does anyone know of any other illness or disease that this could be? Sorry I do not have further details. I'd appreciate any help!

Thank you!