RPG Maker Forums

DISCLAIMER: Tech-heavy chatter ahead.

I'm not into hardware support forums (despite me being a certified technician), but I ran into a problem lately, that I couldn't make heads or tails of, so I thought I'd post here in case anyone has seen anything similar.
-First off, I'm writing from the computer that had the problem, so now it's fixed.
-It's not the first occurence.
-I put the system together myself.
-It has worked for 10 years now.
-It is clean and well-maintained.
-It is the system I use regularly for programming in VX.

I say that so that it's clear how much mileage it could have on it, and that the problem is not something that would happen usually. And, that I can handle most problems.

The problem itself is the motherboard (or, the CPU, or, the PSU, or, *something*) not coming online when it should, at the time of powering up the system.
It happened once, randomly, after I had shut down *and* unplugged the system the night before.
It stayed dark for nearly 14 hours, then I plugged it back in, pressed the power button...... nothing happened.
The PSU came online, the motherboard stand-by LED was on, the keyboard LEDs flashed, but the system wasn't booting.
Hit the reset button.... nothing.
Powered down, and back up.... nothing.
Powered down completely, took the system apart, put it back together..... nothing.
There, I started to freak out.

I went back to the manual and checked every connection.
Took the BIOS battery out, followed a BIOS reset procedure, put everything back,...... nothing.
Stripped everything down and just left the CPU.
Powered back on, and the motherboard started beeping.... that, I knew it was a sign of a missing memory module (which I had stripped)
So, with that, the assembly of MB-CPU-RAM was working.... from there, I plugged everything back, and ran resets every time to double check,.... everything checked out.
That was the first time.
The second time, I simply set it to reset from the OS, and it didn't came back online.... it just stayed there, unable to boot up. It hadn't been a cold restart, just a software restart.
I hit the reset button, and it came back online.... no idea why.
Third time, some time ago... shutdown, wait period of a couple of hours, power back on,... nothing.
Pieced it apart, always observing step-by-step troubleshooting, and eventually it came back online. I have no idea how.

Last time, yesterday, powered down after regular use, took the graphics card out to clean it, put it back in (didn't take me more than ten minutes), powered up,...... nothing.... completely comatose.

I know about error beeps, I know about drops in voltage, and I know about faulty contacts..... nothing fitted.
I spent about 12 hours (last night, and this morning) trying to fix it.
Only when I went to the last resource of taking absolutely everything apart and putting it back together, did it work.
I just can't figure out what the hell is wrong.
It's not a CPU problem, otherwise the computer would have frozen somewhere along the time of use, and that's not the case. It did freeze many times before, but always came back up after a reset.
It's not a PSU problem, because, everything checks out.
It's not a BIOS problem, because, the system boots.
It's not a memory problem, because, the warning system beeps if I remove it.
It's not a GPU problem, because, it's new!
and it's not a motherboard problem, because, the self-diagnostic checks out even before I push the power button!

the build in question is an Asus p5sd2x, with a Pentium 4 CPU, and 2048MB DDR2 RAM (used to be 2560MB, but the other module seems to have died a while ago)
HDDs, GPUs and other devices don't matter, because even taking them all out didn't fix the system.
it just seems to have a will of it's own!