The router should run 24 hours a day without an overheating problem. I presume it does not sit in a window facing south or anything.
My 585 is always on and can go for weeks without any problem so if you had it from Zen contact them about a replacement.
Ken
Zen Active 8000
By the time a family pays off the mortgage for a home in the suburbs, the home isn't home,and the suburbs aren't suburbs.