My mid-2011 iMac 27" suddenly started spontaneously rebooting over and over. As it booted, it looked like this: It seems like a video card issue, right? Especially because it would look like this even before the OS started to load. On a lark I booted from an external drive, and it came up fine, without those bars. I haven't had a chance to really troubleshoot it yet, but is there any way this is just a software issue? Any ideas how to troubleshoot it? Thanks!