i am having the exact same problem, i have the 7970 reference though, bios flashed to GHZ, when i run it at 1190/1820 @ 1.25v i can play normally, but at random times, the clocks go down to 500/900, and i get like 1-5 fps out of the game, until my system fails, and i have to restart. The same is happening at 1050/1500 @ 1.25v, however it happens not that often. It's pretty annoying. My power slider is also +20. I would really like to know what is causing my card to downclock and never clock back normal clocks again until i restart. It is only happening in C3.
Edit: Also you should check your VRM temps, when those are too high, the gpu will downclock to not get damaged. Peak temps may be up to 110°C on those. However if they are >100°C for a long time you may want to get a better fan. Mine are at ~78°C max after 5 hours of playing, so vrm temps are not the reason why my card clocks down. However maybe this is your problem. With programs like gpu-z you can check your vrm temps.