Well, at this point.  It is tough to say what could be the problem.  

I would search the hardware forums about my mobo and see if others are having the same problem.  I would also think about formatting and rebuilding the entire computer with as little hardware as possible to see if I could identify what is casuing the probelm.

On the flip side it may be something easy that we are overlooking because we can't see the computer and don't know its history first hand.  Rebuilding it may be overkill.  

I would rerun both computer testing programs and see if they can spot a problem.  There might be a setting to run the  Burn Test for a longer amount of time.  This may help find the probelm.

It is a tough call.

I have built many computers that all work well, but I do own one that is problematic.  It  uses rambus ram and used to reboot freguently.  I could never figure the problem.  I ended up underclocking the ram and bus and now it is better.  If I push this computer too hard it will reboot.  I gave up on trying to fix it.  In hindsight I should have replaced the motherboard when the warranty was still good.  

The difference between my computer and yours is, mine was always like this and yours was not.  

I believe there is hope to stop your computer from random reboots.

And once a again, Good luck.