About two weeks ago, I had a sudden drop in gaming performance on my 27" iMac with ATI Radeon HD 5750 graphics. While before I could play games without any problems at high graphical settings, suddenly playing Diablo3 for example became unbearable with around 10FPS at the lowest possible settings. Oddly enough I simultaneously started having similar problems when playing games on my Windows Bootcamp installation on the same machine.
I am not noticing any other problems besides those when gaming. Rebooting the machine does not help at all.
What could be the problem and how can I solve this?