Computer randomly shutting down, possible PSU problem?

[spades72]

I've just put myself a new comp together with some parts cannibalized from my old computer, loaded Windows XP pro and started getting the better part of my drivers installed when my computer just turns off. Power to the board is still there and I can start up immediately but after a short while it turns off again. No lockups, no BSOD, no nothing, almost like someone yanked the cord out and put it back in. At first I thought was overloading my rails but after rewiring and making sure things were distributed the same problem occurs.
My specs are:

ASUS A8N-E (bios 1013)
Enermax EG701P-VE (600W, albeit a bit old)
2GB Corasair XMS3200C (4 x 512MB DIMMs)
2 x Hitachi 150GB SATA I
Geforce 7900GT 512MB
DVD rom
DVD burner
X-Fi

I've checked the rail voltages to the 12V, 3.3V and 5V and all of them are withing reason (atleast 5%) and CPU temp stays in the 40C with a high of 48C (last I've seen anyways).
Right now the only problem I can see is that I'm using a 20-pin PSU connector with a 20-pin to 24-pin adapter to the mobo, other than that I'm out of ideas. I don't mind buying another PSU just that I'd rather make sure its the PSU before I shell out the cash for it.
Thanks in advance.

 

[GalvanizedYankee]

If your not on a laptop atm, pull that PSU and see if all is well when using it.


...Galvanized

 

[spades72]

I've pulled PSU out and I can't see anything externally wrong with it, even if I look inside through the fan mounting I can't find and visible signs of burns and what not. Even after my computer suddenly shuts off the PSU fan still runs like its suppose to (its suppose to keep running until the PSU gets below 40C or so it claims). Unfortanately I dont' have a spare PSU to test the board either.

*Edit*
The PSU doesn't get hot to the touch either and the rear fan stops spinning bout 30-35seconds after shutdown.

 

[mechBgon]

1) You got the PCI-Express power cable hitched up on your video card via an adapter, right? And the ATX12V cable up at the upper-rear corner of the board?

2) Do the shutdowns seem to have a random nature, or do they always occur at the same point? Do they occur only when starting Windows, or do they occur also if you just let the computer sit at the BIOS screen?

 

[spades72]

Yes I have the PCI-E power cable hitched up to the video card via an adapter and the ATX12V in the right place. As for the shutdowns its always after a certain time after running the computer (haven't tried letting it sit in BIOS) and each time after the shutdown the intervals get shorter and shorter until I leave it alone for a while which makes me think its an overheating issue but my CPU and case temp always hovers in the mid 40s at most (48C was the highest i've seen for my CPU as of now).