TCoville I regret to inform you that your motherboard has contracted the infamous "Only 1 stick of ram" sickness.
My motherboard is doing the same thing (mine started 2 days after I built it)
currently there is no cure.
Some people say upping memory voltage in BIOS, or MANUALLY setting memory timing in BIOS works.
Either have worked for me.
I have been researching day in and day out for the last week striaght trying to find a solution to this problem. I have brought it to computer stores, I have asked many computer techs, I have asked asus, and AMD, and still I am empty handed.
I created a thread about this issue in the GENERAL MOTHERBOARD form.
(The greatest unsolved computer problem in the history of computers)
If I ever solve it, I will post the answer there.