What is the cause of a "phantom itch? " it comes and goes in random parts of the body, and there's no visible irritation (rash, redness, etc.).

Normal. Everybody gets an occasional itch. Just watch tv and you see everybody scratching. Itch is an irritation of sensory nerves and is an abnormal minute electrical discharge you feel as itch. It is nothing to worry about. Your family doc can see you and discuss it if it becomes more severe.