I Left my Mac running since and had zero further pixels flashing green or purple.
IT would usually have been 1/3 covered in coloured squares By now.
I Also upgraded to yosamite. But it was doing it since half way through Mavericks Life span.
I Don't think it was the OS. I believe the programs (one or the other) have a scripting flaw (intentional or otherwise) that leak kernel resources until the video cars is overwhelmed and freezes while everything else goes on in the background. The mouse cursor would also still move but would be unable to click on.
Google my solution people. i cant remember where i posted how to fix it.
Fix.
Was delete Sophos and Little Snitch. or similar. recent programs. they were stuffing it all up. Runniungsfor 3 days now since. NO MORE SQUARES!