Macbook screen Freezes and becomes Pixilated

Bascially All of suddon my screen will do all 'pixelly' and then the computer will freeze... This has happened a few times now.. and is a bit conerning along with my computer freezing quite often.. I should also add that this happened while I am using the thunderbolt adapter to use two displays. I have a picture from my phone BTW:

Does any1 know what this means?