Well...what ATI card/chip is installed and in question?
System model/manufacturer or motherboard manufacturer/model info would be useful.
I guess I don't see any "error", just an adjustment made automatically by some ATI software.
Since we are talking video, I would have to assume
...that one of two things happened: Your video card was overworked and started to overheat or yur drivers are not the best ones you can use.
We need more info, I think.