1) make sure you have the latest BIOS. If needed head over to www.asus.com.tw
and find a newer BIOS
2) make sure the "etc" that you've turned off also includes legacy USB
3) try a different power supply.
4) if available on your board, enable "spread spectrum" on the cpu and memory.
5) you may try upping the voltage on the memory just a *tad*. btw, what type of memory are you using?
I actually had something very very similar on one of my Linux setups (I think it was my intel based MSI board). Somehow I got it resolved. (my system also didn't completely shut down, but I changed/installed something and now it will). I actually think you may want acpi on. I'd disable APM though. These are just ideas and I hope one of them helps.