Do you have any of your custom BIOS settings written
down somewhere ?
Since you updated the BIOS, you could try clearing CMOS.
There will usually be a jumper, like "CLRRTC". You must
remove all power, before using the jumper. Unplug the
computer. Consult the manual for any additional details.
Once the jumper is put back the way it was, power up,
enter the BIOS, and restore the settings.
*******
When I had a stability problem with my Core2 system,
I found that entering the BIOS, and bumping Vnb (Northbridge
voltage, the chip that talks to memory), that seemed to help.
It only took an increase of one step, to go from occasional
memory errors, to none at all (even after Prime95 all night).
Your architecture is different, but you might still have
some useful controls in the BIOS.
One other thing I'd experiment with, is operating any
"whizzy" hardware features, in manual mode. For example, if
you had "Digi+ VRM", if that could be disabled, I'd run it
in a fixed operating mode. And see if that helps or not.
I doubt the VT-x setting made that much difference. It
shouldn't have anything to do with the operating
conditions for the memory DIMMs.
I was a bit surprised at my stability problem. The
system was flawless for months, then one day I was running
several heavy duty things at the same time, and I started
getting memory errors. I loaded memtest86+, and sure enough,
could see problems there as well. I tried reseating the DIMMs,
in case it was a contact problem. Didn't help. Then I started
fooling with the voltages, and on a hunch, headed for Vnb
instead of VDimm. And just as quickly as my memory problem
had shown up, it was gone again.
I can only conclude, that somehow, either the voltage regulator
had drifted out of spec (unlikely), or, the silicon in the
Northbridge had a parametric shift. But I hadn't been
"torturing" the Northbridge - it was operating at pretty
Plain Jane settings. And yet, I started getting errors.
Tweak it - if no satisfaction, you've likely still got
a little warranty time left. The Asus warranty may be
three years, but it's measured from the date stamped
on the box. The serial number, has two characters as the
lead digits, and those specify year and month. Like "3A"
would be manufactured in 2013 October. (I think this is
documented on the Asus site somewhere.) And my three years
would be measured from there. When I purchased my current
Asus motherboard, it had been sitting in a warehouse for
1.5 years, so my three year warranty was basically cut in
half (only 1.5 years left).
Paul