Hi,
I have to say this, even if I am a year behind the thread:
My 3.3 V rail had suddenly started to hover around 3.002 V, even going down as far as 2.96 V according to ASUS Probe.
It was the only problem. 5 V was very close to 5 V (and exActly 5 V when CPU usage was ~100% !), 12 & vcore also 11.9 / 1.74
The upshot is:
The advice to clean the motherboard is GOOD ADVICE. TRY IT FIRST.
I cleaned out my machine with a paint brush and some good ol' lung air (cough cough). I even cleaned inside the PSU.
Well, when I powered back on, the 3.3 V rail was reading 3.232!
I must have got rid of some volt-sucking dust somewhere. (and the case was not even as dusty this time as I have seen it before)
So, I say it (shout it) again: CLEAN YOUR CASE OUT FIRST, then decide whether you need a new PSU !
Edit: Oh yeah, after cleaning, 12V=12.4, vcore=1.74-1.75, so these also rose a bit. 5 V still hanging around 4.8 when idle.