System troubleshooting, help.

So I'm currently working with a customer who seems to have very bad luck.

I've built a rig for them in the past; and that kept having BSODs which then somehow eventually killed the GPU as well. Turned out the HDD controller (was it NB or SB) died, and so did the GPU when it started crashing its drivers even in 2D.

Anyway this time round, they're experiencing a completely different problem.
Occasionally, the PC refuses to start.
When that happens, a weird chain of events occur.

The moment A/C power is connected, all the fans in the system spin up; the DVD drive works and we can hear the hard disk spinning up and then maintaining speed. All this without the need to even hit the power button on the case. But the rig does not boot. All the peripherals just start up and stay that way. HDD lights don't blink and the power light doesn't come on either. Hitting the power button when this happens yields no result.

When it finally decides to stop doing that and boot up properly (i.e. no fans turning when the A/C is connected, and it goes to POST when the power button is pushed), it works fine for awhile and then decides to randomly freeze up and cut supply to the keyboard and mouse.
The keyboard is a generic Lenovo keyboard connected via PS/2 and the mouse is a generic USB mouse.
The Num Lock light which is left on by default, goes off; and the red sensor light on the bottom of the mouse as well as the decorative blue lights on the side of it all go off as well.
A hard reset is the only way to get it out of the freeze.

Not sure what is going on. The RAM is proven working; the motherboard has been RMAed recently; and the CPU is fresh from RMA. PSU has been tested and ruled out of foul play (well, mostly). And its unlikely that the GPU, CPU, HDD or DVD is triggering this.
Could be the case; but then when I'm around this thing absolutely refuses to misbehave.

Specs are as follows:
Phenom II X2 550 + M4A78LT-M LE
2x 4GB KVR
Sapphire 5670 512D5
WD5000AAKX
LG SATA DVD RW
FSP Hexa 500W

While I was there, I did all I could.

It refused to lock up or not start in my presence. The voltages were checked with an F-115, all are fine. Only lacking an O-scope to check ripple but then again this isn't an iffy PSU, so ripple should be entirely out of the question.
A visual inspection of the PSU reveals no bloated or blown caps whatsoever.

I am inclined to think that the motherboard is at fault; given that the system not only locks up but cuts power to all USB devices.

More troubleshooting will have to be done; but it'll be hard given that it doesn't want to misbehave when I'm trying to make it lock up or not start.

I'll look for that when I pick up the rig this weekend. Will post here if abnormal.