Keyboard works, however black screen when awakening?

Hi,

It's only started recently, say the last 2 weeks, after I updated to 10.8.5. I use extra monitors at home and at work. I have a 30 inch at home and a dual screen via a usb display link at work. They both have the same problem.

What happens is when I put my computer to sleep and then try and wake it up, say an hour later, on one of my external monitors in clam all I get is a blank black screen, and then the monitor turns into sleep mode as I'm guessing it is not primary. Before I used to be able to plug it in and it could switch with no problems at all.

When I unplugg the monitor and open up my mac book thinking the screen will be one, it is blank also. I get no respone from the keyboard, external mouse, track or anything. What I have to end up doing is holding down the power button to kill it and boot it back up and then it works fine.

I've removed all my peripherals thinking that might be something, and still nothing. I'm racking my brain on what it could be. I've reset the SMC and PRAM, and still nothing.

Does anyone have anything else I can do or try? Have you come across this? Is there something I need to change / update?