I didn't see this posted after using search but I saw it in bug reports and mostly with people running Windows 7.
I have a i7 960/12GB RAM/6870 1GB GPU and I still got it. Until I went into my screen saver/power settings and changed the profile of my power savings to "High Performance".
If it's set to the other standard once (balanced and such) your cpu will power down itself from ~3ghz to ~1ghz.
If anyone is having that issue give that a try and see if you still have it :)