This is most likely the intel technology speed step witch reduce the processor speed in idle time to reduce the electric bills
Just check your processor under full load if it switches to 2660 then you are OK. Note this technology can be disabled from the bios.
Or may be a bios issue . Just update your bios and it will work fine .
I hate the speed step technology! I want my computer to be fast, even when I'm word processing. I want my computer to be fast even when I'm not on it. Who likes a lagging screensaver?