Dude, dust inside your case.
I've got a Leadtek 6800GT AGP and started to have some stability issues with it (though it wasn't actually heat related, the game just doesn't like NV drivers) and noticed my load temps were hitting into the 90s, so I opened up my case and guess what? All my fans were cloged up with dust. My idle temp dropped from 73ish to 60 after just cleaning all the poo off of stuff.
Give it a whirl. If that doesn't help enough, try putting some AS5 inbetween your stock cooling solution and your card. That's what I did for my Leadtek and it has helped A LOT in keeping it cooler. If that fails, go out and get an NVSilencer. Though removing the heatsink will void your warrenty, so do so at your own risk.