Well after a few days of this here is what I have determined:

1. 9 of 10 startups are failures. No signal, no bios and no USB power after 10 seconds. Everything spins and whirs, though.
2. Of the startups that DO work, it usually stalls somewhere between bios-windows and the screen goes blank. Then suddenly the machine restarts and loads normally.
3. Once Windows is fully loaded, the GPU spins up at full speed for about 5 seconds, then everything seems normal.
4. I can run graphic intensive programs all day without any issues.
5. Hibernation also doesn't work. The computer doesn't wake up after.
6. Restarts DO work and seem to never fail to boot.

Almost seems like an issue with power getting to the GPU and/or PCI-E slot?

It seems a bit like a problem with the PCI-E slot, but I'm not convinced. I would think if the PCI-E slot was getting no power it would just boot up normally on the onboard video, and if it was getting partial power (enough to detect a card in the slot) you'd get an error. And it also seems weird that that would cause hibernation not to work. Plus most of the power to a GPU comes from the PCI-E power connector, not the slot itself. Do you have another PCI-E power connector on your PSU that you can try? Or even just another PSU you can try a bit of open-case troubleshooting with.

At this point it almost sounds like a motherboard problem to me, but it's always difficult to say for sure. I would think a PCI-E slot problem would manifest itself during operation and not just during boot.

Seeing as it's a platinum Corsair PSU and a sh###y ASrock mobo, I'm gonna go with the motherboard..

Ok, I've also determined I cannot startup without pressing the graphic card down in it's slot first - AND I had the screen flicker just now, resulting in a reboot. It does seem like a PCI-E problem.

Matthew wrote:Plus most of the power to a GPU comes from the PCI-E power connector, not the slot itself.

That depends on the level of card, actually. I'm running an HD4670 on this old fossil, and it doesn't even have a PCI-E power connector...it draws all the required power through the PCI-E slot itself. Unless I've missed something, I don't think Hunter has mentioned which is the case for him.

I suppose that's true. I just rather assumed that since he's running graphics-intensive software that the card was probably powerful enough to at least have a PCI-E power connector. But you're right, that may not be the case.

It's a Radeon HD 5800 series, pretty power hungry. I bought a Corsair especially for it. I am fairly convinced the problem is my motherboard, but hard to say for sure.

Most likely. It doesn't really sound like a PSU problem because it runs fine once it gets booted. And not wanting to wake up sounds a lot like a motherboard problem as well.

Damn ASrock boards..

Anyone got a spare PCIE 2.0 card I can test with?

Wouldn't it be cheaper to buy a new one than to pay the ship-back cost?

Seems the PCI-E slot is the problem - I've done a few startups while applying pressure to the card and it stays on until I release it. What I don't understand is how it works at all. Like right now.

There might be a worn-down / burnt-out connection pin. It could also be throwing errors the whole time, and it just keeps on keeping on after startup (but fails and quits during).

As my mum would like to say.. It's a complete mystery!