Ok so I have a mid 2011 iMac that is wanting to make me throw it out the window! Currently I have an extra monitor hooked up but this issues happens without it hooked up as well yet sometimes will occur on both screens or just the main iMac screen sometimes. I have upgraded the OS, ran all updates, and reset SMC and PRAM as well with no fixing this issue. Every so often I will get flashing pixels on the screen. Different colors, sometimes they just stick on and sometimes they flash. Most of the time when this happens I can just right click and 'change desktop background' and change pictures and then the flashing pixels disappear. I assume it's an issue with the video card (AMD Radeon HD 6970 1GB) but before I try and find another/newer video card that will work with this machine I'm curious if anyone else has ran into this issue before. I have a video of the issue as well and can upload it later if it helps.