I've just been on with Gigabyte support about this issue on my Z87-HD3 board.
I had 2 pairs of Corsair Vengeance LP RAM, and I bought some more - 2 new pairs of the same brand/model (cheeky bid on ebay worked!) so I put one of the new pairs in, and instant system instability. Processes would crash, occasional BSODs, etc.
Turns out that the XMP profiles on the RAM can change. My 3 pairs of RAM all looked the same, same brand, model, etc but one was rev 2.12, another rev 7.4 and my original pair rev 8.0
Turns out the rev 2 and 7 sticks shows XMP 1.2, and the rev 8 sticks showed XMP 1.3 - which had different timings. The tRC was different at 33 (whereas support showed the ram running with tRC of 41).
In my case, I swapped the sticks about and they all showed XMP 1.2 and stability has been perfect since.
So I have to say its not gigabytes fault (in my case), more Corsair's for changing the spec of their ram without any notification. It could be this is the same issue for other RAM manufacturers, though I see a few posters are using Corsair too.
To tell, get CPU-Z and run it, on the SPD tab you can see the timings and XMP version used for each RAM slot.
