Do you live in the US? In the Midwest, Northeast, or South?
It's because your video card is overheating. It's probably old, and time to be replaced. Does the fan on it actually rotate? Clean it out, that might help.
Now that it is getting warmer the video card has to work harder to keep itself cool. If your cooling fan is old, it may be dying. If you are seeing graphics issues like locked screens, horrible stuttering, bad refreshes, or no 3D viewing area at all, then this is likely your problem.
Go here and get a new one for $150: