- What PSU are you using?
- Did you at any time overclock any of the components.
I'd suggest you make sure the PC is stone-cold stock, and if the problem persists we'll look harder at memory, graphics, and power.
- Uninstall Asus (and any other) utilities that can overclock your PC.
- Clear CMOS (use mobo feature or pull plug, remove battery, press the case power switch several times, grab a cup of coffee, come back and replace battery, plug into wall.)
- Immediately boot into BIOS, load (optimized) defaults. Make ONLY changes that are absolutely necessary for your PC to run correctly, if any. Save and boot into WIndows.
You can go further to eliminate some other potential probelms:
- Download CPUID Hardware Monitor, Prime95, and Furmark from this site:
http://forums.tweaktown.com/gigabyte/30530-latest-overclocking-programs-system-info-benchmarking-stability-tools.html
- Check for heat and memory problems by running Prime95 for 15 minutes, Furmark Torture Test for 10 minutes, then both at same time for 5 minutes. Stop the test if at any time a core temp exceeds 75C and appears to want to keep climbing (ie, 77C limit), gpu temp 85C (87C limit).
If Prime95 can't continue because of an error (rounding) there's a memory problem.
It may be that your BIOS is interpreting a failure as an overclocking error, and is restoring a saved BIOS. This would explain why your selected boot drive changes.