Bad signal with DP/mDP with Dell U2414h

I bought a new Dell XPS desktop and a new Dell U2414h monitor in the middle of February 2015. A couple of weeks later, the monitor would not work at full resolution (1900x1080). I called tech support, and after a few reboots of the computer, the problem disappeared.

Unfortunately, the problem returned on May 5 or May 6. On May 8, I used Dell tech support chat and my nightmare began. In less than an hour, the tech rep and I diagnosed the problem as the DP cable. However, instead of planning on sending me a replacement cable, the tech rep asked to "try a few things." I made the mistake of saying okay. The tech rep spent two hours reinstalling drivers that I had already reinstalled twice myself! After the third go-around, the tech rep asked whether there was anything "important" on my computer! Yeah, think about that. What was the tech rep planning, reformatting my hard drive in an attempt to fix a malfunctioning DP cable? It didn't make any sense, and my display was still malfunctioning!

It seems obvious in retrospect that instead of honoring their warranty, Dell was going to start destroying the information on my hard drive until I gave up. So, I gave up. I bought a cable and used the recovery tool to do a system restore. (I could not change the resolution in the state that Windows had been left in by the tech support rep.)