I have an iMac OS X 10.6.8. I think it's 6 or 7 years old. The screen breaks up every now and again where pictures become like a thread of colors, or a line runs across the screen. It has twice had a grey screen drop down from the top and sometimes it freezes and the only way to unfreeze it is to turn it off. In the last 2 days the normally square cornered windows have become curved and fuzzy and the frames are white with a yellow splash. Can anybody tell me what's going on? Is it dying? Is there something I can do?