Crashing between phases

Have just started having crashes between the phases - when I jump back from a battle into my own arena. My screen will try to change colour(?) then get stuck on a solid black (or red, I've had both) blank screen. I can hear the game continuing on, but I can't see anything bar the solid colour screen and I can't even CTRL+ALT+DEL out.

Only started happening today.

Any ideas?