Screensaver crashing in background?

Hi there

I have a iMac 12,1 and I've had it for a couple of years. It came with Snow Leopard and I have upgraded it to Lion and then to Mountain Lion. Since I upgraded to ML I have this weird problem. I leave my machine on all week, so when I come into work, I wake it from sleep, unlock the screensaver, check my email, then turn to my PC to do some AD admin. When I turn back to my Mac, the screen is black. Sometimes if I put it to sleep and wake it again, I can get into the Finder but it's still black. Using my trackpad I can switch from workspace to workspace but the screen is still black. I can quit my open apps but cannot shutdown or reboot, I have to hold in the power button to switch it off. I have even formatted my hard drive and done a clean installation of ML but it hasn't helped. Before the rebuild, the only crash logs I could find that might have had a bearing on this mentioned Quicktime crashing.

What has alleviated the problem (i.e. it only happens once or twice a week as opposed to every day) was that I changed my screensaver from one of the slideshow ones to the arabesque one. None of my users have complained of a similar issue. I also use the Creative Suite and one of the first signs that my machine is going to go into this state is that Bridge crashes and will not re-open. I'm also the only Mac user running Apple Remote Desktop, but I don't think that it has any relation to the issue.

Has anyone had a similar issue and managed to resolve it? I am a techie, so I have done all the obvious stuff like rebuilt, repaired permissions and updated the OS, etc.