Hang on Sam will find vide strip down of it for you step by step.
Follow the link here it shows you how to get the heat sink off the card, and how to clean the old paste off the gpu die, and the back side of the heat sink touching the gpu . and how to apply the thermal grease. The shroud should come off easy with a undoing a few screws if you look. give it a good dust out with a paint brush if there are mats of thick dust blocking the cooling fins with a brush. http://www.youtube.com/watch?v=I4cNWiWYdj8
Good luck i hope it sorts it, if not bake it as a last resort.