Been using a XL2411T 144 hz BenQ monitor for a while now and was able to use 120 and 140 hz on my Radeon HD5870 but since putting the RX 480 in the other day, 60 hz is the highest available in desktop display settings, although Catalyst Control Settings/Radeon Additional Settings is saying 120hz is the maxiumum supported but I can't change it. Used dual link Dvi cord on my old card and same cord with displayport adapter to card on this new one, tried hdmi and had the same problem.
MOBO: Gigabyte M4A89GTD Pro/usb3
CPU: AMD Phenom(tm) II X6 1055T Processor
GPU: XFX Radeon RX 480 Black OC 8GB