I see your card is pretty high on the hierarchy list of video cards, so it should work ok. Tom's Hardware GPU List

But even with my ATI HD 5850, my GPU monitor shows it working at 99% on full settings in Xsyon.

Have you tried turning off your overclocking on the GPU card and monitoring the results? Overclocking by it's nature taxes the card, causing it to run hotter when stressed. If you haven't compensated for that stress with extra cooling provisions, then it will run hotter.

Running a circuit hotter than normal also can cause premature aging, which is one reason I don't overclock with out using serious cooling provisions.