Using the latest FGLRX release from AMD ( AMD Catalyst™ 14.12 Proprietary Ubuntu 14.04 x86_64 12/8/2014), the system still hangs at least once per day using a R7-260, so I downgraded back to the open source driver. Your results may differ for the 6750.
You also mention re-installing Ubuntu, which shouldn't be necessary. If you can get a desktop, you can uninstall the drivers using Synaptic, or open a command shell and execute the AMd uninstall script. If the system wont boot in graphical mode, use the recovery mode and un-install the fglrx drivers via dpkg (if installed as a package) or the ATi uninstall script (which I think was in /usr/share/ATI).