Flickering crash constantly

Today I was doing some editing on FCP for about 3 hours and everything was fine. I saved and shut down to go have dinner. When I came back and booted up, my screen started flickering wildly and everything was frozen. I had to hold the power button to restart. 10 minutes later same thing. Then again and again. I noticed Firefox updated itself tonight and that is the only thing that has happened. I haven't installed anything else in weeks. Any idea of what is causing this?

I am using the latest version of Lion on a Mac Pro with 12gb of Ram. Like I said, no problems at all until late tonight. Now it is happening constantly since I quit working earlier. Sometimes it is after 2 minutes, sometimes after 20. The screen just starts to flicker and everything freezes. I don't know what to do or look for. I don't understand why it would happen all of a sudden when everything has been fine.

Thanks for any help.

Sounds like a Firmware or Hardware problem. Check all your Software Updates are run, Apple Menu.

If they are, then get it to a Apple Service Centre ASAP, running it like this could damage your files!