In simplified terms your graphics is basically two chips in one. The VGA portion which is basic and will work from the BIOS without the aid of any additional drivers and then the other portion, which does require drivers.
So, the graphics card/chip could be defective and still look OK in safe mode. You may have noticed, safe mode only gives you VGA or SVGA mode @256 colors.
I recommend you try updating the graphic drivers and see if that clears up the problem.
If the corruption only occurs after some period of time you may have a GPU (graphics) overheating issue. If the inside of the tower has not been cleaned in a while then blow it out completely using canned or compressed air. Don't forget to do the power supply from both end too.
DO NOT use a vacuum cleaner.