...making sure it's the hardware and NOT the OS. Boot Knoppix and *USE* it... Just like you'd do to get the re-boots... If it don't reboot it AIN'T the hardware... if it REBOOTS... then it's the hardware...
Assuming it's the OS (yes I know...) I am wondering when the last time you actually defragmented the drive... I'd schedule it for everynight. Yes, even with NTFS it can get severly fragmented and affect peformance *AND* stability. Don't revert to older drivers... especially with the nVidia stuff... usually it's a good thing... *I* don't believe it's the hardware persay... maybe some BIOS settings are not "optimal" for your setup.
Play around with them, but change one-two at most at a time *UNLESS* you know for sure WHAT it is. Use it, run it for a while, no change... put it back or leave it works either way. Then change some more... use it, if worse chnage it back... better leave it, and continue to tweak. You'd ve surprised how touchy SOME ASUS motherboards can be... Especially the VIA chipset boards... like you have... BUT they also tend to be the BEST boards out the ONCE you tweak em right... (on and ASUS A7V-133 Right NOW) as I have had experience (as has Wade) with Crappy doings on ASUS *UNTIL* you hit that Magic Sweet Spot... then it is typically SMOOTH sailing... for a long while. I STILL am having to tweak an Asus A7V-333 right now... to get it stable... grr... BTW, Auto for Voltages and such on ASUS boards is typically a good thing too... unless Over-Clocking... which you aren't.
I also have GigaByte Motherboards... GA-7VRXP (2), and GA-7VAXP (1)... love em to death too... but they as well require the tweakers touch... but not as much.
Wish it were easier than that... but... you know...