If you suspect it is your graphics drivers I have found Driver Sweeper to be useful. Just download it here and do as it instructs. http://www.guru3d.com/category/driversweeper/
Then download the latest drivers from the ATI site and try that. I've had flaky drivers before from ATI that don't install correctly. It's a good place to start.
just curious what 400W psu do you have.