The conclusion of that post is the XFX HD 7970 has obviously a voltage locked OC range..
Well, maybe this wil help some frustrated OC-Friends:
You can adjust the voltage control with the Sapphire Trixx Tool (MSI Afterburner and other tools won't make it). The option is called "VDDC" and can be set to the max value of 1281mV. To avoid conflicts with Catalyst, deactivate the Overdrive functionality.
With max. volt of 1281 Ive reached 1200mhz vcore and still could go further but I'd rather stay between 1150 - 1200 mhz.
Hopefully I could help and maybe others with that card cheerfully get that nasty voltage issue off their oc plans!