Horizontal lines around cursor

Macbook pro 15" 2.00ghz.

8 horizontal lines have appeared around the mouse cursor, causing a freeze, and now will only start in safe mode. Apple think it might be the graphics card or main board, but would need to replace each part to be sure, this would cost a small fortune. Hardware diagnostics say everything is fine. After a fresh install everything was fine for about a month and then the lines re-appeared. Any ideas?