Late 2009 iMac intel partially black screen graphics help?

Question

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.

Answer 1

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