after 10 mintues with Crysis at 1680, there is no dust
If you are certain that the HSF is clean and on correctly and the paste is good then there is always the possibility that your friend had cooked the card by running it heavily OC'd and has fried the thing.
have you check your idle and in game temperature? maybe you'll have to replace your thermal paste
also YOUR case might not be setup correctly to handle increased temps from the OC card
make sure that your case has enough air flow and also check if the thermal paste is dried up or not..