I have a great 2011 MBP that I decked out with 16GB of RAM, 1 GB Samsung 850 SSD, and run it on my Dell 27" display like a desktop. Lately, I've been having this issue where strange little sprite-type blocks are appearing randomly. They are primarily purple and green and they flash in little clusters in random places. When I first noticed it, I rebooted and it seemed to go away for a day or so. Yesterday I noticed it again so I rebooted which didn't resolve it. I unplugged the displayport cable and it seemed to clear up for about 2 minutes then started up again. Anyone able to help me troubleshoot this without my having to take it to the Apple store? I keep the machine on a cooling plate so it shouldn't be overheating. I disabled automatic graphics switching a while back since I don't use the battery.