I'm pretty sure my OC is stable: 15min Gputool stable + 10min furmark stable. :/
I lowered the clock, same thing.
But the problem seems to be my CPU actually: I OCed the NB to 2600 and now I get 17K
I'll continue to study the problem, because for some reason my mobo's NB is unoverclockable (anything above the default values will make the computer crash during Intel Burn Test). So I can't just live the NB that high.