As I've been sick and tired of the instabilities of OpenWrt on two Fritz!Box 4040 and the Linksys EA8300, I thought a give the ZyXEL NBG6817 a try and wow this thing just works stable as I was used to from the old Buffalo WZR-HP-AG300H. I can only assume that this issue might be related to the amount of RAM. The unstable devices had just 265 MB and I could get them down easily by just watching a YouTube video, downloading somehting with wget, git or Steam. The ZyXEL has 512 MB and I think this is the reason why it is so stable (but as it has another chipset, it might be related as well). The old Buffalo router was stable from my point of view because it could not download more than 25 MB/s with its weak CPU.
I wasted a lot of time, money and nerves, finally the ZyXEL is the solution for me and I can say after 2 weeks uptime that it is worth every penny.