Black rectangle flashing in upper-left corner of the screen

[echeij]

I have Ubuntu 12.10 with unity and for some reason sometimes when I boot up I have a small black rectangle flickering on the screen. The bad thing is that if I suspend my computer, the screens turn of at the very last moment, just before the computer itself suspends. When I boot up after that, the screens won't turn on and I have to reboot. So the screens aren't just black, they don't get any signal

I have an ATI Radeon HD 7670 with two monitors and I use the proprietary ATI drivers. I also tried the post-release updates but they had the same problem.

I added a screenshot below

I searched around for quite some time but I couldn't find anything so I hope somebody can help me.