The attachment clearly indicates that the router considers the reboot to be cause by a power blackout. I see two possible reasons - either you indeed have issues with the stability of your power, or the supply in the router is failing.
A simple test is to connect your router to a provably correct UPS and see if the problem goes away. If it does, you should investigate the power installation and delivery with your electricians. If, however, the problem persists although the UPS is capable of providing a truly uninterrupted power then the power supply in your router is probably failing and will have to be replaced.