Hi all, Getting sort of desperate here. I've got an mid 2011 iMac 27 (12,2) that's suffering from massive crashing. I run time machine so I'm not too concerned about data. Running Mavericks right now for an OS What's happening is that from time to time (occurring more frequently now) the screen will freak out and go to any number of different states - either pink with lines, white with lines, flashing like a strobe etc. Usually, the machine will not re-boot with a normal start after such an event. Here's what I've done so far: Safe boot - works maybe 50% of the time. Running a verify disk tells me that the disk is fine. I repair it anyway. There's usually some permission errors. Re-boot usually works. I have 16G of memory so I've been swapping it out to see if that's the problem. 8G was from Apple and 8 from somewhere else. Samsung is one of the brands and I forget the other. Seems to crash irrespective of which ram is in there. I ran the Apple Hardware Test and it freezes at about 75% in of the probing hardware. When it locks up the space behind the mouse pointer turns red. The only things I haven't done so far is to wipe the disk and re-install OS X. It looks like because of the age of my machine, I'll have to go Lion and then upgrade to Yosemite - is that correct? I'm going to hold off a little bit on that to make sure I can do everything right. Any other advice? Due to the problem with the hardware test, I'm worried that something is screwed up with my Logic board or perhaps the GPU. When the screen crashes, I can still hear the hard disk crunching away in the background but it never recovers. Sometimes the machine will re-start on it's own if I leave it alone long enough after such an event. Please help!! Thanks!