Is my motherboard dead or is it the ram

is my motherbaord dead or is it my ram??

well i have 6gb ram 1 x 2gb and 1 x 4gb, and a gigabyte GA-MA790XT UD4P (it says the rest of the specs at the bottom)

ok well when i have all 6gb in or either just my 4gb my computer freezes a few second after i get to my descktop, i cant use CTRL - ALT - DEL and my mouse doesnt move its like im viewing a image in full screen, so i have to hard reset using the reset button on my case.

now heres the weird thing when i only use my 2gb everything works well no bsod freezes or anything, this makes me think its the ram, but 6gb ram ran fine in safe mode and the 4gb ran fine in another computer (asrock n68c-ucc phenom x3 720be nvidia 7024).

aslo it workerd fine for about a month and 3weeks with the mobo and the 6gb ram installed

to conclude the 2gb workes fine the 6gb (2gb+4gb freezes and the same with only the 4gb, does this mean my mobo is dead or its the ram please help so i know what to purchase either a new mobo or ram