My company has been having the same problems with all the Dell Latitude E6420, E6520 & E6430 laptops that have upgraded to the 15.6.1 driver. I originally upgraded the drivers directly from Dell's site when the problem started. I also tried using the driver directly from Intel's site with the same results. So we can rule out the theory of using the system manufacturer driver as a fix for this.
Intel, please fully investigate and provide a fix for this widespread network driver issue.