It's the power saving feature, I believe for AMD it's called Cool & Quiet from what I remember. When you fire up a stress test your core clock will jump up to what you clocked it to in the BIOS.
You can also disable Cool & Quiet in the BIOS. I personally don't find a need for it. Some really care about energy saving, though.
Originally Posted by ComradeNF
Just ran Prime95 and it was still at 3.4GHz
. Lemme try restarting my PC.
Heh, you sure you applied your changes before you exited the BIOS? Bump the multi back up to 18x, f10, save and exit.