I've read that the GTX 280's had a problem with the IHS not making good contact with the core. My PNY GTX 280 also heats up and I had another one before and it ran perfectly. The only solution I can think of is to lap the IHS, I think trying to remove it would be too dangerous. But then you'd have to find a thin copper plate to compensate for the lapped IHS, and lapping it all the way to the core is easily going to take you 12hs+ since you have to be careful not to knock anything out.

It sucks but those are your choices, I'd insist on another RMA or atleast trade it for a GTX 260 which don't seem to have this problem, or at least it isn't that common.