First off let me say I don't know anything about macs, because I'm a PC user.
However, in the world of PCs I'd claim either what qCzar said about a failing gpu, or (and I'd say it looks more likely) that your GPU is overheating to the point of crashing your entire system.
Could the vents have become clogged with dust etc etc?
Or maybe you just have it on too long doing gpu intensive things (I was considering a mini for 3d modelling in modo - now I'm curious as to whether it'd cook the thing 😀)
Chief 🙂