Its odd that it is the first slot/channel. I wasn't aware the computer would even boot without that channel being functional.
I'm sure you have done this already, but as I thought I'd run into the same problem with my son's computer I'll pass this along. It was all operator error.
I've built well over 2 dozen "from scratch" builds in the past few years, so I thought I was impervious to doing "dumb" things. I'd taken one of his DIMMs out to see what he had and then the trouble started. Long story short (i.e. much swapping around, taking memory from my comp, etc. etc)... the DIMMs simply were not seating properly in the memory slot. I repeatedly (not just once, but several times) did not properly seat one or more of the memory sticks. Asus has changed their memory slots so they only have one lever that holds them in place. Even though I was locking the lever, I was not getting the opposite end down far enough. (My eyes aren't what they used to be either.)
tldr: Just check and be sure your getting the memory card down all the way.
Memtest run?
Here's hoping its really something trivial.......
