I would think this was conflict of some kind more like a plug in than hardware.
However if it just started happening it might be something to do with a setting either in the drivers for the card or Illustrator or the system
you can try toreset your settings by holding down the control, alt and shift keys (all 3 together) while starting Illustrator.
That will reset the settings or you can move or delete the settings folder if that does not help, sometimes something else in the folder becomes corrupt.
Also this can be a conflict with a font, if you have installed or updated software recentlyI would suspect that software or font. Have you used any free fonts recently in a document?
Here is how you find and remove the settings foder
1. Quit Illustrator.
2. Rename the AIPrefs file (for example, to AIPrefs.old) in the Users/(user)/AppData/Roaming/Adobe/Adobe Illustrator CS3 Settings folder.
3. Start Illustrator. Illustrator creates a new preferences file.
4. you can also just trash this folder if the new preferences work out fine. Also if this does not work then move the entire Adobe Illustrator CS3 Settings folder to a different location like your desktop for temporary storage Illustrator will create a new folder with all the elements required within the folder and those settings will be fresh default.
However if there is a conflict or hardware issues this will not probably help.
if you do not find the in this location do a search for it.
Also if you have moved the computer or installed any hardware in your desktop it could be the card is no longer seated well or the card is not quite right or the display could be funny or going funny. But I do not think so.
It was a corrupted AiPrefs file!
I simply removed the old one from the Users/[user]/appdata/roaming/adobe/illustrator cs4 settings folder and restarted CS4 and it was all sorted.
Thank you for your help.
That easy you're welcome.