i7-4770k will not idle below 37 multiplier. Any ideas?

[JEebu]

Stepping 3 and Revision C0

Asus Z87-A motherboard, BIOS '2103'; low power states enabled

The processor used to use the entire multiplier range of 8 to 39 (799MHz to 3.9GHz). Now, at 0% CPU usage it will hold at multiplier 37 and go no lower.

Any suggestions on what to check?

[JEebu · Accepted solution]

Found the issue was nothing with the processor or motherboard. Windows 7 had changed the power option from Balanced to High Performance. Problem resolved after putting it back to Balanced.