my laptop display go blurred after scrolling

At first the display is just all right, but after running sometime, especially when I scroll the screen quickly with the mouse wheel or scrolling the right side bar, my laptop display will go blurred (gray background with black dots and lines), firstly at the top of the display, bars and menus (when I slide my mouse over them, they may appear again from the gray blurry), flash in web pages; at this stage it's not serve, I can still read, or if I let it rest for about ten minutes, it may recover all normal.

However if I keep using my laptop after the blurry screen appears, it will get more server, the whole display will go blurred, and the words will change into black blocks, now it cannot be identified at all. It will not recover again after resting even for an hour.

This problem appears almost every time when I run my computer, at first it's okay for about two hours, then the problem arise.....

My system: Ubuntu 10.4,
Gateway LT31: CPU: AMD L310, 1G memory, self-included ATI video card.