if its fully powering off..and by powering off you mean shutting down and not losing network connections..then something is amiss with the device and its either needing a part replaced out or the whole thing needs replaced.

I had the same issue when first setting up the internet as it would reboot every 2 hours 40 to 45 mins.  I resolved the issue by doing a factory reset.  Connection status was always very good or excellent when it rebooted.  Only thing I can pin point is maybe some loop in attempting to update firmware?  Been rock solid after the factory reset.

It could be as simple as the power transformer. If the power cord has a transformer on it that could be bad or going bad. Of course it could be a problem in the gateway itself. IF there is a heat related issue causing it to shut down then replacing it would be important. Get a replacement.