What type of Adapter do you have?
You need a Dual DVI to reach 120Hz refresh: (from wikipedia: http://en.wikipedia.org/wiki/Digital_Visual_Interface
The DVI specification mandates a maximum pixel clock frequency of 165 MHz. With a single DVI link, the highest supported standard resolution is 2.75 megapixels (including blanking interval) at 60 Hz refresh. For practical purposes, this allows a maximum screen resolution at 60 Hz of 1,915 × 1,436 pixels (standard 4:3 ratio), 1,854 × 1,483 pixels (5:4 ratio), or 2,098 × 1,311 (widescreen 16:10 ratio).
To support display devices requiring higher video bandwidth, there is provision for a dual DVI link. A dual link doubles the number of TMDS pairs, effectively doubling video bandwidth at a given pixel clock frequency. The DVI specification mandates how the dual link may be used. All display modes that use a pixel clock below 165 MHz, and have at most 24 bits per pixel, are required to use single-link mode. All modes that require more than 24 bits per pixel, and/or 165 MHz pixel clock frequency must use dual-link mode. In dual-link mode, and in modes using 24 bits per pixel or less, the transmitter stripes pixel data across both links; each sequential video pixel is transmitted on alternate links. In modes with color depth greater than 24 bits per pixel, the second link carries the least significant bits of each pixel.
My guess is that your DisplayPort adapter is only a passive single link. You'll need a double link DVI adapter (rare) to run 120Hz.
Not sure about config and root, but check permissions on your files and make sure your personnal login can read those.