I have a 27" iMac, Mid 2011 (3.4Ghz i7, AMD Radeon HD 6970M 2GB) and I am experiencing the following intermittent issues, which are solved each time by a restart:
When watching video with either Quick Look or Quicktime the video will break up and be overlaid with either large blocks or lines of pixelated colour often pink and green.
Often when this happens the type on the menu bar will also break up and become sort of faded looking and hard to read. Desktop items may also break up becoming unrecognisable.
Here are some images of what this looks like:
I am wondering if this is a hardware or software issue?
When I first got the iMac it had Snow Leopard installed. Not long after I upgraded to Lion (now running 10.7.1). As far as I can recall, this did not happen under Snow Leopard, but I did not run the machine for that long under Snow Leopard before upgrading. I have not experienced the freezing issue that other iMac owners have seen, and software update has not recommended the patch for the freezing issue, so I have not installed it.