Thanks guys. I got it to work.
Setting all 4 cores to the same multiplier did the trick. I had the default option, i.e. all cores on auto, which caused the spiking.
@Force: I tried turbo off yesterday and the whole thing run on 800 MHz for a couple of seconds. Well that's at least what CPU-Z showed me, so i switched it back on.
Btw, my CPU cooler is a real monster. There is no need to check the temps!
Bios temp of the CPU is 30° celsius. I let the core voltage on auto, so it can suck the power it needs, as that seems to change quite often.