It seems that after a few minutes of being in a game or benchmark the frame rates start to take a dive. For instance I ran Unigine Valley 3 times and it was lower each time. I've noticed performing lowering over time in games as well, when I'm not benchmarking.
The weird thing is restarting the application fixes it, until some time has passed again.
It's not being caused by overheating. CPU stays 45-55C during use in games/benchmarks and GPU is 65-70C. Both are well within specification.