I have the exact same issue, my less than 60 day old 2011L-RM repeatedly would loose network access and would have to be rebooted 20 times a day, the device was still powered on and I could access through winbox, which I thought was odd, but any traffic that was to be routed through failed. I setup the device to do a supout but unfortunately it never did no matter what I had tried.
I took a RB951G-2HnD, as that's all we had for a replacement at the time, and imported the export from the RM and it worked flawlessly for 3 days, so that eliminated human error on settings. We then took the RM and did a series of test downgrading firmware, upgrading ROS, net-installing different ROS versions down to 5.17 to see if the issues would appear solved, and of course, to my dismay, the issue remained.
Ive always hesitated on using the latest version of anything as it normally has a whole set of new problems, but I was desperate and didn't want to RMA our brand new device, so i tried ROS 6.0. Now it seems to have the issue resolved, whatever it may be. I just hope 6.0 is stable enough to deploy as a PPPoE Concentrator.
Been running 3 days now on ROS 6.0 and no hiccups, knock on wood!