Try putting the side back on. The coolers sometimes rely on airflow to keep the thing cool - having a big void at the side (which is what you have when you've pulled the cover) means there's no airflow other than convection which often isn't enough to keep things cool...
Have you tried the 84.21 drivers they are the most stable one's I've found but I can't say I've tried them on a PCI card only on an AGP & a PCI-E but they might work for you.
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