My 2011 iMac is experiencing some kind of hardware failure. I would appreciate some advice on how to narrow it down so I can determine whether a repair or new iMac is in my future. System: 2011 27" iMac 2.7 GHz i5, 12GB RAM, 1TB spinny drive + 128GB SSD that holds the OS and apps, Radeon HD 6770M 512MB graphics. Yes, I've done SMC reset and all that usual stuff. Symptoms: 1. Graphical glitches like this. These occur daily, and in different areas but usually only on the top half of the display (unless symptom #2). They flicker in and out, and if I drag a window over them they change -- either they disappear for awhile or they get worse. It seems random. 2. Extreme instances of #1 where the graphical glitches take over almost the entire screen, the mouse cursor turns into a giant chunk of multicolored pixels, and then the computer freezes -- I have to hard reboot. This occurs 2 to 3 times a month. 3. The computer will turn off and then not turn back on until I unplug and re-plug the power cord a couple times. This has only happened twice in the 6 months or so it's been having the other symptoms. Any suggestions? Thanks!