I experienced the same issue as mentioned in this post with my U2172HM (A00 manufactured in December 2012, Purchased on the 14th of June 2013)
U2713HM "Lines" on display with flickering
http://en.community.dell.com/support-forums/peripherals/f/3529/p/19495965/20318974.aspx#20318974
The horizontal lines and flickering problem started severely, even when nothing is connected to the monitor. Then it kind of reduced towards to the center area of the monitor and then became visible just on mixed colour back ground only. i.e. Solid RGB, Back, White were ok but on the Built-in Diagnostics's grey background it was very obvious.
But just when I was about to give up, it went back to normal. this whole thing lasted about 2-3 hours. What I had been doing during this time includes self-testing, built-in diagnostics, factory reseting, lcd conditioning, unplugging and re-plugging.
Any idea what had happened there. I am thinking if this monitor is worth keeping or shall I get it replaced. Hard to prove it is faulty though, now it is totally normal again.
