GPU Overheating while idle

I have a VisionTek Radeon HD 4870 card that, in recent months, has begun to overheat while at idle, requiring me to manually set the GPU fan speed at 70% or higher (I had thought that the normal ATI Catalyst Control software had a function to automatically speed up/down the fan to prevent overheating, but if it does the feature isn't working).

Any ideas on what might be causing my card to overheat while it's doing nothing?

the heatsink might be full of dust...

what kind of temps are you getting?

the plastic shroud should come off easily enough, just some screws... but dont loosen the screws on the gpu back bracket or the heatsink itself will come off and then it'll need some more tinkering, reapplying paste and all that...