OK folks, I guess I am a little guilty, so apologies for any misunderstandings.
With regard to voltage tolerances, if you look over here ...
ATX 2.01 Spec [pdf]
you'll see that voltage specs are quoted at +/- 5% (3.3 volts @ +/- 4%, but close enough).
However, if you now look over here ...
ATX 2.03 Spec [pdf]
you can see that some of the tolerances have been changed to +/- 10%.
Needless to say I was looking at the older spec when I made my post, but its not really relevent, as most devices will operate outside these ranges (especially on the '+' side) without much complaint - overclockers do this all the time.
With regard to voltage and current not necessarily rising together, well thats true, because it all depends on the resistance. Remember that simple litte formula they taught you at high school (voltage = current * resistance), well that shows that for any given resistance (notice I'm making an assumption that the resistance, stays the same, then current does in fact increase with voltage.
Now, given that the problem computer in question is as stated running normally, though unstable, and the the PSU is working (we are in windows running asus probe) and power is being supplied, the expected current draw would be as normal. Note that due to the way switching PSUs work to maintain output voltages, and relating to the issues raised in this thread (voltage @ 13.2 volts does not necessarily mean that the PSU is supplying enough current to the computer) ...
1) If the PSU was NOT able to supply enough current, i.e. the load was demanding more than the PSU could provide, then the output voltage would decrease and NOT be maintained at 13.2 volts. This is typically what happens when you use a PSU that is of too low wattage rating for a PC - the computer won't be able to power up at all.
2) The only other way that 13.2 volts could be present, with low current output, would be if the load was very high (low resistance). This is why lower rated PSUs can be used in smaller PCs that have lower power requirements.
Finally, as we already know that the PC in question has booted into windows and all the peripherals and components are running (i.e. the load is normal), then the measured voltage of 13.2 volts suggests that the PSU is meeting all the demands for current and that is why the voltage is able to remain high.
Try putting in some high current drain devices (low resistance - e.g. short circuit) and you are certain to see the voltage drop followed shortly after by the PC shutting down (assuming that it had already booted before we add the extra load).
Also note that the current demand is greatest at start up, therefore given that the PC has booted into windows it would seem reasonable to believe that the PSU is in fact easily capable of supplying the power required (to maintain its current state).
Now I am NOT perfect, and I DO get things wrong from time to time, but mostly I try to be as accurate as I can. Its true that the atx specs I refered to were older versions (his PSU might very well be atx 2.01), but it doesn't change the reasoning behind my comments, which are valid and still stand.
If anyone believes otherwise, then please feel free to post your reasoning here so others may comment. Hope this helps clarify things a little, so don't be afraid to post if you disagree and wish to say so.
One last thing, there is one case where I can see that the PSU in question, is unable to supply enough power, despite having 13.2 volts output, and that is if there is some device present that can suddenly increase the load on the PSU by a significant amount, such that the output voltage would momentarily drop below the required threshold. But then that should be noticeable, and would not be random as it could be replicated quite easily. Also, its hard to imagine any device that could achieve this, though its not impossible.