I have this same problem, I haven't had it for a while though until today. I was using firefox at the time and just as I clicked on the youtube link from google's homepage the screen suddenly goes black, and then the log in screen appears. I don't know if it helps, but I'm using Ubuntu 12.04 LTS 64-bit (the latest release) with up-to-date amd video drivers.
I have this same problem, I haven't had it for a while though until today. I was using firefox at the time and just as I clicked on the youtube link from google's homepage the screen suddenly goes black, and then the log in screen appears. I don't know if it helps, but I'm using Ubuntu 12.04 LTS 64-bit (the latest release) with up-to-date amd video drivers.