late 2009 iMac partially black screen graphics help

I have a late 2009 27 inch imac which started to get this issue:- webpages will have black where images should be, open window boxes will be black, drop down menu's either black or multicoloured and can't be read, mouse over icons on dock makes them black, text documents is all black- however the wallpaper is normal and borders of the webpages etc...
Booted using the apple hardware test cd- short test and long result resulted in no hardware issues however only long test fixes the black issue for a day before it returns again.
What is the issue, do i need to replace the graphics card. I believe there is alot of dust inside so could this be the issue? I also installed the smc fancontroll and set speeds of around 2000rpm with average temps under 55'c. Before smc speeds were default to 1000 rpm and temp highest was 60'c.
I have also tried switch on imac when fully cold, but straight away the issue is present even though temp is low of 30'c. so far the only fix is to long test using the apple hardware test cd.
Please give informed help as i am pretty useless with regards to programs etc..having been a pc user before this imac.
incidently the graphics card is ati Radeon HD 4850 512 MB.

The best way to test the graphics chipset on your iMac is to attach an external monitor or TV to it. You'll need the proper adapter and cable in order to do so, but that will tell you if it's a graphics problem or related to something else. If the external monitor displays the same problem, then you'll know it's graphics related.

About the graphics card in your iMac... I'm not sure if it can be replaced without replacing the entire logic board. Some older models of iMac had a removable graphics card, others did not. You may be able to find out by going to iFixit: The free repair manual - iFixit and look up the repair procedures for your model iMac.