I have a GA-Z77X-UD5H as well, and a month ago bought a U2713HM.
When I first plugged it in and hooked it up I used my existing DVI cable that was previously driving my 20" LCD and had the same "fuzzy"/"buzzy" picture at 2560x, but it was perfectly sharp and nice at 1920 and lower.
I fixed it by swapping to the DVI cable that Dell had included in the box - which was a fatter cable, and I have since learned was Dual-Link DVI which allows for the needed additional bandwidth that the high 2560x1440 resolution requires. http://en.wikipedia....e#Dual-link_DVI
Give the cable a try, hopefully that's all it is.
TomGo to the full post