I use Mikrotik as PPPoE server (with freeradius) but...
I use 1Gb lan cards, and I think that because of that this happens:
After few minutes, link (to ISP) fall to 0, after few seconds, it back, but problem with memory start. When memory become 0, routeros restart itself.
I think that images are enough.
I use routeros 2.9.27
With routeros 2.9.42 I do not have problems (but also I do not have 1Gb lan cards).