So, I have several RB750GL's running meta routers, most running 5.2
I went to install a new one and upgraded it to 5.7 and it would randomly reboot a couple times a day.
So I downgraded to 5.6 and had the same thing happened.
Then I downgraded to 5.5 and it seems stable. (read: up more that 1 day)
Just FYI.