That's why it's best to use DRAM from full sets, not mix sets, generally with low end DRAM like 1066, 1333 iit will often play together , you have low freq and loose timings, I've had6 sticks of 1866 before in the 1366 socket and they were fine, of course needed a good OC on the CPU to help the MC carry the sticks and additional voltage, running 6 sticks at 1333 or 1600 shouldn't be an issue, though only time I've seen problems was with CPUs that had a weak MC