Exact same problem here.
This is now my third U3415W who does that thing. The first one did it only when connected to DisplayPort. Not Mini-DisplayPort. Then I got it replaced but the second one had some issues with the touch buttons. Now I have a third which randomly goes black for one to two seconds when a computer is connected to the Mini DisplayPort. It doesn't seem to do it when a computer is connected to the DisplayPort or HDMI. Alas I'm using a Surface Pro 3 and dock, which only have Mini DisplayPort and the DisplayPort is connected to a Mac Pro. I could find a Mini DisplayPort to HDMI-adapter, but in my opinion it should work on all inputs.
Regards
TGMadsen