This is quite common on laptops with Nvidia or ATI graphics chips, and is normally due to repeated overheating. How old is your laptop? If it is no longer in warranty, then your repair options can be quite costly. I had a Packard Bell SB86 which after 2 years use suddenly gave me the same problem. Check out this link: www.fixyourlaptopyourself.com. It explains how you can fix a range of laptops with this type of problem yourself, and without too much hassle - unusual technique, but it worked for me.