The manual states on the specifications page in the manual that 1680x1050 @120Hz is the optimum resolution. It may not support all resolutions, and revert back to 60hz.
Try checking to see if you have the latest drivers installed on the computer.
Only refresh rate with my 2233rz monitor available at monitor properties/CCC is 60hz on all resolutions. I have hp pavilion dv6 laptop with mobility radeon hd 4650 and 10.7 catalyst drivers. 2233rz is connected to my laptop HDMI port with DVI-D -> HDMI adapter. I have tested this adapter and my DVI-D cable on my laptop with another LCD-monitor and it succesfully runs on 85hz.