I would first see that you have the newest bios updates..looks like there been 13 for this mb.
i would use cpu-z and see what the ram is running in speed and timing. if the mb after the update has the speed set fine and the timming to what the ram is rated for. try running memtest again. if it fails see if there a voltage changes for the dram.i would bump it if it at 1.5 to 1.6 or 1.65 and see if it fails. if it does reset the voltage back and then move the ram. i would put the two larger rams in the black slots and the smaller ram in the blue. if that how they are now then put the two larger ram in the back and the two smaller ram in the front.
if the rams dont have the same timing..is one 9-9-9-24 and the other is 7-7-7-24. the faster ram may not be slowing down.