Hard to say.
I had a Netgear router that would reboot itself around 5 seconds after the internet got connected, I was able to resolve it by switching off Wireless as it appeared to be a power issue. I replaced the power supply and all was well for a while but it began rebooting again. I flashed the firmware with a customised version and all has been well since.
Electronics should not reboot themselves at a predefined interval unless some sort of corruption is going on. Try factory resetting the router if you are able and see if that resolves anything.