Most cards have a single and dual link DVI port. You MUST be connected to the dual link one to use 120Hz. The single link port cannot support that signal.
The monitor may also just be broke
I have tried both ports, and it is a friends monitor, i regularly used it when i set my machine up there and ran 120 fine, and my settup has not changed since then, im now able to boot with 1 monitor thanks to a driver update but everything else ive tried doesnt seem to work, and after some tweaking i found the max it will give me is 77hz