I was having this issue (or one that sounds remarkably similar). I seem to recall doing something similar that mitigated the problem, however it eventually returned just as strong as before. The 1.33NA firmware has stopped this from happening (as well as making the speed issues happen less), however it introduced a steaming hunk they call Securespot which is apparently not disabled when you've disabled it. (see other threads for discussion).
There is apparently an issue with some NTP servers causing the router to reboot, you could try selecting a different one and see if that solves the problem.
Alternatively, if you've recently bought this router I would suggest returning it and getting a router by a different brand. If it's too late to return it might I suggest investing in a heavy blunt object and forcefully interface it with your router and then buy a new one.