Hi and welcome to the Gigabyte Forum.
You have already done a lot of the checking but at the moment we don't seem to be that much further forward.
When you said you checked the RAM did you use Memtest86+ or the beta version for the X58 chipset?
How many times did you run the loop before changing modules?
Did you run the modules seperately?
What are the temperatures of your system like in normal running?
Which processor do you have installed?
Sorry for all the questions but there will probably be even more to come.