Look at the time frame on when the last complete boot up time was and the time frame when it reboots. You might enable System, Critical, Informational and Warning log options and see if you can find anything that says something when this happens.
The probably is probably WiFi congestion on 2.4Ghz that effects the router rather a HW problem. I presume that if you shut OFF the WiFi radio and just used wired, the router probably wont reboot.
I recommend that you get your devices upgraded to 5Ghz. Devices like PCs, laptops and game consoles can be upgrade to use 5Ghz WiFi with by internally or externally upgrading the HW.