The OP is several years old and applies to the XPS 15, circa 2011. The solution for that old hardware on Windows 7 was to get either a Dell miniport display cable or one from Amazon as posted in this thread. I used both and they were flawless. The problem was the only miniport display cable widely available at that time was from Apple and it worked in certain conditions, but not all.
My miniport display cable came with the monitor from Dell, I don't think they supplied an apple cable.
like I said it worked for 2 months and now its not recognising the display.
I think it is some conflict between the intel and the NVidia and dell display manager.
I have got to try another type of cable yet
I had same issue with my XPS 15 9550 and Dell UltraSharp U2412M.
I just bought HDMI <> DVI-D cable.
And issue has been solved.