Charles is right...your room may be cool, but if your case is in an area like the corner under a desk it might not be getting enough air flow to it...I know those little cubbies can be the worst.
You might look into getting something like the NV Silencer even though really, I figured as massive as that leadtek heatsink is that it would do better.....maybe the springs suck and it's too heavy for them to firmly press it against the GPU....
Suppose as a cheaper alternative you could put some AS5 on it and see if that brings down the temps, but even that wouldn't be drastic or anything.