Go back to fixed, imo. I'm not positive on other board manufacturer's but the Asus z87 boards I've tried cannot cap the cache and core voltage in any other voltage mode. For ASUS bios it's Manual mode for yours it must be fixed.
The trick is to use Windows power management settings in control panel and set the minimum CPU usage to 1%. Then it will downclock as normal in my experience.
A lot of windows processes run for several minutes in the background after boot and windows login, and will keep the processor running at full speed. this is normal.
What about parked cores. Just disable core parking if it is an issue.