Thanks for the reply.
The BIOS is already latest I think from 10/2012.
I ran memtest86+ a few hours without errors and I'm usually running the system without GUI most of the time and had no problems.
Trying your tip with 1033mhz and disabling powersave mode. I'll report back in a bit
edit: The powersaving sounds promising, because in most cases I don't see a trace of the freeze in the logs.
edit2: So far so good, 1hour in! I'm letting a video playlist play, the cpu is chilling at 8% with 45°C.
I'm really excited right now, because I spent many hours trying to solve this problem, trying different drivers, settings and distributions. And researching it on the internet was too specific.
I'll let it run for a few more hours and hopefully I might be eternally grateful, fritsch!