Thanks for answering. If I did know I wouldn't use drivers of course. Now I'm not sure what is wrong. I checked graphic card in Windows XP- no problem at all. In this case I think something was messed up with system, not HW and maybe GPU's MHz is correct.
If I would like to reinstall drivers for Windows 7 , from scratch I mean, what should be done? I reinstalled them few times but with no success.