Are you by any chance sending log messages to a syslog server? I do not have experience with this model router. But for the routers that I do have experience with I would suggest connecting to the console port of the router and capture any console messages that are generated. If the router was stable until the code update it does suggest that there may be some issue with the new code (and a memory leak would be a likely possibility). But any log messages produced leading up to the reboot are deleted in the boot process and are not available after the router comes back up. This is why the console log might be helpful.
The other suggestion is that if this router has an active maintenance contract with Cisco that you could open a case with Cisco TAC.
HTH
Rick
