Apple have released an EFI firmware update for their Intel Mac Pros, which:

This update fixes several issues to improve the stability of Mac Pro (Early 2008) computers.

To complete the firmware update process, please follow the instructions in the updater application (/Applications/Utilities/Mac Pro EFI Firmware Update.app). The updater will launch automatically when the Installer closes.

The update can be installed through Software Update, or downloaded from here.