I realize this is an old thread, however, I ran into a similar problem and finally got it to work, so I figured it was worth posting. Error code 15 refers to the North bridge startup which makes it sound like a CPU related chip set issue. I reseated the CPU, checked power going to the CPU, etc. - the sorts of things you'd do when running into a CPU issue. Eventually I started swapping out memory and finally got it to work with with a single 2 GB stick (1333) in slot one only. Initially I was using two Corsair 8 GB sticks which incidentally seem to be OK.
The moral of the story is that you have to start with a very vanilla memory configuration with minimal memory (I started with 8 GB, switched to 4 GB and ended up with a 2 GB stick) to be able to access the BIOS to tweak it for the larger memory configuration.