The root cause of the watchdog timeout can be hardware- or software-related. Here are the common symptoms through which you can identify the source of the problem:
If a router that has been properly operational for months suddenly starts to reload every 20 minutes, or if it continuously reboots and you can no longer access it, the problem is most likely a hardware-related issue. This is also the case if a new module has been installed recently, and the router crashes by watchdog timeout afterwards.
If the router starts to crash after a configuration change or a change in the Cisco IOS software version, it is probably a software-related issue.
For more information refer to the following url:
http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_tech_note09186a00800f67db.shtml