could be an overheating issue. given that you have a HD4250, you've probably had that laptop for a year? maybe the vents are getting dusty. if you or any of your friends have a laptop cooler that might help.
On the other hand, I would suggest going to the Sony website, find your laptop's model, and see if they have a driver update for your video card. if not, maybe check out the AMD website (which definately has newer drivers). however, I would go to Sony first as the AMD drivers may or may not play well with the rest of your components