strange screen error

[radix]

Either heat, or an issue with the card. What you are seeing is called artifacting. This is common with overclocking and has to do with errors on the shaders. Try running something like FurMark and see if you can replicate something similar. It may not look the same everytime, but unless you underclock you will continue to see this.

It could also be your computer hating you for running SL3.