Have the same problem on my D4250WYKH with G.Skill Ripjaws F3-1866C10D-8GRSL 2 x 4 GB, i know that mine is 1866mhz but have the exact same problem, tested both memory modules in both sockets separately with no errors in Memtestx86, and that would say that single channel is working fine, but when both are in so it runs dual channel it fails in Memtestx86.
Really hope this could be fixed somehow