I just added a U2414H to my existing U2312HM. They're driven by an AMD FirePro V3900 (Windows 7 x64) with DVI and DP ports and are connected like so:
Computer V3900 DP --> Dell DP to mDP cable --> U2414H mDP in
U2414H DP out --> BizLink DP to DP cable --> U2312HM DP in
They are both turned on end, portrait-style, and joined to make one Eyefinity group for a single, large desktop. Sometimes they'll work fine all day. Other times, especially after being turned off (with the computer) and then on again at a later time, the 2312 will go into--and stay in--sleep mode and the screen on the 2414 will turn on and off. If this happens, it usually starts right when the login screen comes up, though it has ocurred long after startup. When blinking, the screen-off duration is usually only a second or two, the on duration varies dramatically from a few seconds to several minutes. If I put the computer to sleep and then wake it up, that will sometimes bring the 2312 back on and stop the 2414 from blinking, but not always, and usually not for very long if it does.
I tried reverting back to the original DVI connection for the 2312 and used the DP - DP cable to attach the 2414 with no cable linking the monitors to each other. With these connections, the 2313 will stay on with no problem. The 2414 will still occasionally blink as before. Curiously, if I leave the 2312 connected via DVI but switch the cable connecting the computer to the 2414 from the BizLink DP - DP to the Dell-supplied DP - mDP, I lose resolution. The maximum (and only) resolution option drops to 960x680 across both screens! Does anyone have any ideas what's going on?
One note: I tried to follow Chris M's "Instructions before you post a monitor question". I restored factory defaults and performed self-tests on both monitors but I could only do the Built-In Diagnostic on the 2312. I couldn't get "buttons" 1 and 4 depressed on the 2414 in such a way that the screen would go grey. I kept trying for the entire self-test period but could only manage to get the input selection to appear. It usually takes me 3-5 tries at the power "button" to turn the monitor on or off, so I wasn't too surprised. Frustrated, yes... but not surprised.
Thanks