I purchased DIR 855 two weeks ago and the router is restarting every hour or so.
The router restarts with a wired computer connected.
My system:
Windows 7 Home Premium 64 bits
Version 1.21 router firmware, Hardware A2
Broadcom NetLink Gigabit Ethernet, driver 12.4.0.3
If you need more info, just ask.
Kind regards, Petter