Problem with brightness on graphic card Intel 945GM

Hello, I’ve seen the other thread where there are my same problems, but I don’t have solved my problem. I’ve an Acer Aspire 5710 and the graphic card in an integrated Intel 945GM. The problem is that I can’t set the brightness, when I set high brightness after some seconds it become low.
How I can do to solve this problem?? Please help me. Thank you