This problem occurs under Windows 8.1 (x64), while in bios everything looks fine. The bus speed should be ~100mhz but it keeps floating around 95mhz on idle, and 96mhz under 100% load and because of that cpu cant reach maximum speed. I tried to change power plan to high performance, and disabled all power saving options in bios (C3, C6/7, eist). With no effect. Can it be caused by some Intel drivers?
What's interesting Intel XTU does not see the lower clock.
Where do you see the bus speed fluctuating or what tool are you using to monitor the CPU? I would assume that this is an issue with the software that you are using; the XTU would give more accurate information.
I found an article reporting this issue with CPU-Z: