If I understand you correctly, this is happening with new computer and new router, but also happened with a previous computer and router.
I can only think of three reasons.
A. One of the 'other devices' you mention is telling the router to reboot. What are the other devices? Is one of them a computer that is logged into the router control panel software?
B. The mains socket/circuit that you are using for the new computer (and its predecessor) has a loose connection causing interference.
C. There is software on the router (the like of which I have not heard) that has been told to disable the router when a prohibited or unauthorised device tries to connect.