I had the same card, same problem. I think the problem is that tiny HS, well mine had a tiny HS anyways. Also the thermal compund was thickly spread and we all know it should be what? spread very thinly. I took the HS off lapped it real pretty like then applied some of the shacks compund and superglue. That helped a quite a bit actually. Another simple idea may be to have a fan blowing on it, or a rear exhaust fan or attach a 40mm (i think thats the size) to it. Its a fairly old card so i dont think youd want to spend money of a blue orb and the likes so 1st just keep it simple. I suggest just lapping it and applying a thin layer of compund and i know that will help out a bit. But if you could find a replacement for that tiny HS that would be even better.
Im assuming youve got the same card as me with the tiny tiny HS on the GPU, a friend of mine claims to have the same card with a giant HS on it so.....you know.