So i have an alu iMac 2,8Ghz 4GB ram, Radeon HD 2600 256mb, OS X 10.5.6. Sometimes after i wake it up from sleep i get screens like the one attached. If i aim right i can manage to restart it by guessing where the apple logo is, but mostly I have to hard-restart it. (hold on/off button). Once restarted, problem is solved for quite some time. What the hell is happening, and how do I solve this?