Too few information..Do you monitor the card's temp? CPU and memory clock is default? Underclock the memory(use msi afteburner) to about 320 to 350 mhz(try different values each card is different) to decrease the temp. Then you can try to overclock the gpu to get better MH/s values.
Try few versions of ATI SDK and graphic drivers too.
The temperature is normal, max 65C, but the activity is 98%. I don't plan to overclock at all, I am satisfied with my Mhash/s, and I have a 3 year warranty that I don't want to risk. And my computer works normally, I can use the internet, it has about 12 hours since it started mining and it didn't freeze, but like I said, it freezes in about 1 second when I start any youtube video. The weird thing is that the sound of the video freezes a few seconds later.