mountain lion graphic issues/artifacts after waking from sleep

[seniorchang]

I just updated my late 2007 iMac from Lion to Mountain Lion and whenever I wake it from sleep mode the screen is distorted and full of artifacts, like it crashed or something. I can still see the mouse moving but I can't really read anything since everything is distorted and full of little squares.
I have to restart the imac using a shortcut then everything is back to normal.


Any ideas what is causing this?

 

[seniorchang]

If anyone is interested here is the solution:

I installed the ATI drivers from Snow Leopard and everything went back to normal.