Reading the forums, a number of folks have reported this issue with many different computers.
I bought a U3425w and hooked it up with my HP system and had this problem.
Upon advise I rebuilt and moved to a DELL T3500; same problem.
I was running a new nvidia GTX 750Ti and was told it was a problem card...
Got a replacement card (at no charge, thank you EVGA); same problem
Put in a nvidia Quadro FX 1800; similar problem, but less often ?
Swapped cables, tried HDMI, DP. MiniDP; same problem.
IMHI, it's NOT an Apple related issue...
I just read the idea that DELL's default DP 1.2 might be the issue and am trying that tonight