Just built got a new computer and started running into problems very similar to the ones in this thread. After trying all of the suggestions and not finding a solution, I started to dig a bit more into the hardware and windows operating system to try and understand what was happening.
This error can be described as: Everything is working great and then everything slows down for a while, then everything is great again... rinse and repeat. Depending on the number of things you have going on (overlays, Fraps, shadows, particle effects, high detail) Frame rate fluctuates from 150+ to around 20 frames per second.
Before I get into what I hope is the fix, I can tell you this is not a multi-core, speedstep, or C state issue.
I am fairly certain this has everything to do with the CPU core temperature. There is something on the motherboard, or in the OS that wants to keep my CPU at 37 degrees Celsius. When my cpu is this temp, everything is fantastic. My CPU is running at 3.2ghz and I can turn on every single graphical enhancement in the game with no slowdown. If my cpu goes over 48 degrees Celsius, my cpu clocks back to 2.7ghz and everything slows down until the CPU cools down to 37 degrees. This can take a few seconds or a few minutes. Each speedup or slowdown time period is based how much I am trying to do or not do at any given time.
The "temp" fix: In bios there are a few optional settings under power management. The first one I unchecked was something to do with Fan Speed Control. Once that was unchecked the fan started running full speed. Since I had never heard the fan previously this was sort of a shock, because it kept ramping up until it now sounds like a hair dryer. I started playing the game at this point (trying my best to ignore the fan sound) and everything worked great. No slowdown. But I don't believe this is the fix. What I am doing is overriding the system that would slow down the CPU because it never reaches the temperature that would cause the slowdown.
The real fix: Is still unknown to me. I am now looking into the Intel MEBX system and chipset drivers that are installed in Windows 7. If you have a Dell PC, you can get into the MEBX configuration by pressing <ctrl>+<p> when you see the Dell logo at boot. There is a place to disable this system which will be my next thing to test tonight.
Sorry for the wall of text.
tl;dr -- Your problem may not be a multi-core issue.