Try running a higher memory clock. I have a PowerColor 5870 that doesn't like to have the memory run under 800. I have a couple 5830's that have absolutely no issue running with the memory running at 300. Keep an eye on the wattage draw though. An 850 will work, but you generally don't want to push a PSU past 80% of its capacity. Also as the equipment heats up, it will be less efficient and will draw more power. See what you can do to get some more air to that 5850 running at 85C. It would be much better you could drop it by at least 15C. Run it without a case if you need to.

-SteveA