Often the best solution to this error is to close the programme down, then uninstall the programme completely from your system. Download CCleaner and run a application/ programme scan to make sure all hidden and registry/ log files have been deleted. Restart your computer and re install the client. This could also happen due to stress on the visual memory and visual processing. Try playing the game on moderate graphical settings and you may find it more stable. Also make sure your Direct X9 version is up to date.