Hi Rafal. I'm Greg, a volunteer installation specialist and 8 year WIndows MVP here to help you.
Have you made sure you have the latest Display driver from the PC's Support Downloads webpage, then compare it with the one offered by AMD using their auto-detect utility? http://support.amd.com/us/gpudownload/windows/P...
It's possible the chip is customized by the PC manufacturer which is why we need to always check the PC maker's site first for these drivers, but compare those performance with the chip maker's own driver to see if there's a quality difference.
You can also go over this checklist to make sure the install is set up correctly, optimized for best performance, and any needed repairs get done: http://answers.microsoft.com/en-us/windows/wiki...
I hope this helps. Feel free to ask back any questions and let us know how it goes. I will keep working with you until it's resolved.