It seems like a lot of X58 motherboards have an issue with all six DIMM slots full and memory stability. I've seen a small increase in the DRAM voltage seemed to help solve this issue. I would give that a try and see if that helps out.
The issue with using all the DIMMs goes back to at least the X38 boards. With the x48 boards, you should increase the voltage to the northbridge a bump or two. I suspect the same will solve the problem on the x58 boards, too - but I don't have an x58 board to test it on.