I'm new to this forum but have been folowing this thread (and others like it) for several days now.
In my case, I'm using a GA-EX58A-UD3R (rev2.0) motherboard but am experiencing the same problem.
Something that is interesting, though, is that this is the second one of this particular model which has exhibited the same problem for me (the first was replaced by the dealer after a day or so because of a fault but, during the periods it did work, it had the same problem.)
My new motherboard sometimes shows 12 gb after a cold boot but in most cases, it shows that slot 6 memory is not usable (irrespective of how many times I shuffle different memory boards around, it's always slot #6).
I have checked the motherboard pins but, as per above, this is the second board which has this problem so I don't believe that's the issue. This problem appears to be extensive across a wide range of manufacturers but always with an X58 chipset. I am of the opinion that this is an X58 related issue rather than any hardware fault. I suspect that an updated BIOS may ultimately resolve the problem.
I tried the latest beta (FB11) and immediately after the DMI posting, a message appeared along the lines of "Recovering lost memory size" which gave me the impression that they might finally be on to something with this new BIOS. Unfortunately, it immediately rebooted after this message and did the same thing over and over so I had to revert to FB5.