Graphics are more memory sensitive then text is. Graphics are also video sensitive. You may have no problems with text, yet you can see video problems with the graphics. Some times both graphics and text are affected 1. We had a noisy CPU cooling fan causing graphic problems our computer.
Dust bunnies in the computer can also cause graphic problems.
Cigarette smoke can cause problems with the internal workings of your computer showing up in your graphics. Smoke from your fire place can also cause problems with the internal workings of your computer showing up in your graphics and floppy disk drives. An out of date Video Driver can cause graphic anomalies. You can call your monitor manufacture or check their site on the net for the latest drivers.