I don't really know how long this has been going on, but a few weeks ago I noticed CPU-Z reporting my multiplier as 6 (down from 12.5) when my CPU was under heavy load. To be sure I downloaded and ran several stress tests, yet the multiplier didn't change.
I have tried disabling C1E to no effect. EIST appears in BIOS and I can change it, but the change doesn't stick (it changes back to "default" when I save the BIOS settings and restart; also apps that can enable EIST from within Windows, have the EIST checkbox disabled). I have updated the BIOS to the latest version. I've changed my Windows power plan to maximum performance and set the minimum and maximum CPU states to 100%.
I also tried using cpufrequtils with a LiveDVD of Ubuntu. It informs me that Dynamic Frequency Scaling is disabled, but at least there, my Multiplier is stuck at 12.
I tried clearing the CMOS using the motherboard jumper. If I do that and choose to load the default settings at boot, Windows only detects a single core, but its multiplier DOES change (until I reboot and it's back to 2 cores, multiplier stuck at 6).
When a bios does not support a new CPU, it locks the CPU to a safe multiplier which is 6. As long as your computer boots up without any problems, you can use ThrottleStop to force your CPU to run at its proper speed. You need to enabled EIST / SpeedStep, set the VID and Multiplier values as high as they can go, and the click on the Turn On button in Throttlestop.
If you don't know how to use ThrottleStop, send an email to the guy that wrote the program. It's in the About... box of ThrottleStop. If you can boot up, there's no need to buy a new board.