Router 2811 with IOS 12.3(8)T 8 software installed is getting frequent reboot, there is no issue with power supply.
Is there anything recently changed, like configuration or any IOS upgrade downgrade?
After how much time it gets reboot?
Console into the router and provide the logs of the boot process. It will be good if you collect them when router is rebooting.
in the flash there should be a crashinfo file which if you open it up should explain the reason of the reboot