I'm not absolutely sure what the AMD variant is, but I've noticed many intel boards have a power limit for the processor. APM, however on an intel board you can't disable it and it must be changed to allow the processor to fully open up. There is a long and short duration power limiter on these boards and maybe there is some variant on the 990fx?
Also, those temperatures scare me. Why? Prime95 on blend should easily take any processor to above 40C if not higher. Even undervolted to 1.075v, my 2600k at stock speed hits over 45C in prime95 with my fans at full blast.
Either way, the throttling is almost assured to be some sort of power limiter/power saving feature of the motherboard. Key is to find it.