My Fx-8120 has been under-clocking itself recently. It happens seemingly at random, and I can't find any explanation for it. My first two cores stay at the regular clock speed of 3.1 Ghz, but the other 6 down-clock to 1.4 Ghz. They return to normal after a restart, but not for long. Why is this happening?
It does this to conserve power because it doesn't need to use those cores.
Yeah, Modern CPUs underclock themselves to save power. The only time you'll see all 8 cores at max is when you are running something that needs all that power.
It was doing this while I was doing anything. I only noticed when Assassin's Creed I started running real bad. I changed affinity to the first two cores, and continued on to find they were the only ones running at normal speed, even when the others were being heavily taxed and slowing the game down.
First two cores were the only ones running at normal speed because they were the only ones being used, or the only ones being used enough to need a higher clock. AC is an old game and i highly doubt it uses more than 2 cores. Also, if none of your cores are at 98-100% load then they aren't slowing you down. If the game runs slow it could also be your GPU.