Sorry I mistyped above, it is 2x4 as you stated earlier, and was debating on wether or not go get another 2x4.
But yeah mem test and gaming are fine at 2666 but it simply won't boot at 2666 after being switched on for a period of time and having moderate to high use. The only way to get it to boot is to lower the speed to 2200/2400, before I had made these adjustments anyways as so far 2600 seems quite stable.