iMac distorted display

Hi,

I am having a sporatic issue with my work iMac 27" (Mid 2011). I have found that if I leave it on all weekend, I will find it on Monday looking like this:



As you can see the desktop is distorted (and yes that's my desktop) Also, the video seems to be interlaced. I can usually get it to revert back to normal after unplugging it and letting it sit for awhile. My office is climate controlled at 68-70º, so I can't imagine that it is overheating. If anyone has any thoughts, suggestions or ideas please respond.

Thanks!