Another possible cause is overheating, which may go unnoticed if you do not have the email alerts enabled, although I think this should be logged - this by the way may be a sign of an impending failure.
Shortly before my DNS-323 failed I had daily overtemp shutdowns with the unit sitting on the same shelf (on a 19" comms rack) where it had sat for several years, moving it out of the rack - literally 12" sideways - stopped the shutdowns but the unit died a few weeks later.