Yeah i had the same problem, There should be a hotkey for your startup that lets you choose a boot device for one time that comes up in a sort of classic menu. It should say on the BIOS picture that usually displays when it posts (beeping noise)
It might be a key like F12 or something like that.
Check memory timing and voltages. Sometimes a small bump in memory voltage is needed for stability.
Also if you have access to a different set of memory give it a try.
I had very similar problems with a build. Memtest ran for 4 hours reported no problems. Nothing seemed wrong,except reboots and crashes. Changed the memory from Patriot to Corsair and been rock solid stable from then on.
As Rick suggested i would like you to go into BIOS, Check your RAMS voltage. This shouldn't be too hard. Since RAM DDR3 RAM runs at about 1.6V on [AUTO] your RAM needs 1.65V, Not much of a difference in Voltage but it may fix it.
That's the one. I have the same board. If you have the latest BIOS from ASUS (I didn't check on the that came with mine), you'll have to set it to 1.64 or 1.66, because it goes in increments of .02 and won't accept the odd number, even if you enter it manually.
I'd try 1.64 first, then go to 1.66 only if necessary. According to the advice I was given by my RAM manufacturer (OCZ, I'm having issues with my RAM at the moment as well), a .01 increase over the max won't hurt the CPU or any other internal components, but it still scares me personally.
You also might follow the "Test Your System" steps from this thread. to make sure your system is up to snuff and there are no errors. I had errors when testing with Prime95, which was my first indication that something was wrong.