So I've got four monitors; three Dell U2312HMs (1080p) and an iPad 3 LCD (2048x1536). The iPad LCD is DisplayPort and since there is no way to convert DP to HDMI cheaply and at more than 1080p, I have to use the two DVI ports and HDMI with an HDMI > DVI cable to connect the other three. My graphics card is a GTX 680.

If I have the iPad LCD connected - it doesn't even have to be on - I get a message on the middle monitor (the HDMI > DVI one) saying 'the current refresh rate isn't supported, switch to 60Hz' or some shit like that, even though all four monitors are set to 60Hz. If I turn it off and on again several times sometimes it'll work again, but today it was being a real shitter and kept displaying the message when a game blanked the screens when switching to fullscreen mode.

Could it be:
a) A dodgy HDMI > DVI cable?
a) ii. A fault in the graphics card? I'm guessing it's probably a design flaw, but I have no idea why the other two monitors are fine nor why it worked before but not now...
c) A fault in the monitor?
4) A problem with the power supply?
V) All of the above?

The thing is it wasn't always like this, I've been happily using the iPad LCD with no ill effect whatsoever for a few weeks, but today it was really bad. I don't mind too much since I can use it with my laptop instead (once I figure out why on earth I'd need such a thing) but it kind of sucks as it was supposed to be an accessory monitor for my PC so I could keep an eye on things while I go space trucking in Elite, real trucking in ETS2, sort-of on-foot trucking in DayZ or not trucking at all in one of many racing sims.

Side note: How far are we from having more DP connectors on graphics cards than DVI? I'd quite like that.