Been having this issue quite often as of late. My video card is a Nvidia GeForce GTX 760, and I don't believe it's overclocked as I'm not too savvy about fiddling with my device settings. It seems quite random when it happens too, I can find no rhyme or reason. Heck, last time it happened I was sitting in Sea of Clouds looking at myself and the sky, nothing graphic intensive at all.

One fairly recent change though, as in over the last couple days. Used to be I could just log right back in no problem. Now? It requires a full restart of my computer (Logging out and back in didn't fix it) to log in again.