All hard drives fail eventually, it's usually a gradual process as first one sector becomes damaged, then two and so on. Files mysteriously disappearing is a typical symptom of this. There is no remedy other than replacing the drive. It may not be deteriorating but only testing it will confirm one way or the other.
Back up the drive's contents to another drive ASAP, then test the drive with "SeaTools for Windows": http://www.seagate.com/support/downloads/seatools/