Not used to this so please bear with me. I thought that this might help someone.

I built a system with an i7 on a ASUS z97-p board with a Kingston SSD and Seagate harddrive.

Loaded the correct drivers and Windows 8.1 downloaded from MS

All ok except when lookig at the Performance Monitor it shows Interrupts and DPC sitting at 10-12% CPU hogging one core and forcing the clock frequency to continuous 100%.

I found the MS tools and used xperf to id the offending interrupt and found ACPI was full on continuously.

Looking on the net I saw items relating to the on board Realtek Ethernet and Audio but disabling these had no effect.

Then messed with services and no joy. The weird thing was I found that putting the PC to sleep and then back on again cleared the fault.

Lots of trials and head scratching. Eventually noticed a recurring critical error on event viewer on start up... Event 41, Kernel Power - The system has rebooted without cleanly shutting down first. This error could be caused if the system stopped responding, crashed, or lost power unexpectedly.

Looks to be a startup issue and behold Windows 8 fast start is very different to Win 7 (hence those folk don't have this issue) Fast startup looks to be the cause so switched it off - see Power Options - choose what the power button does

Long story short is that in my case this seems to be the culprit (fingers crossed).