What is your screen resolution, and your 5870 3d clock speeds set at ?
All of your components look worthy of the 5870, so it would most likely be a software issue, such as a bad driver install.
Uninstall ATI drivers through the Control Panel, reboot, run drivercleaner Pro, reboot, then install new ATI 10.1 drivers.
That is the safe way to reinstall drivers. Good luck.