When I was fault finding on m/cs with faults like yours I would remove everything that was not needed to get it into the basic operating mode. So remove all but one of the ram modules, all the sata discs except the one with the OS on it and any other cards you have installed except the video card obviously (unless there's an on board graphics option as well) externally just have the keyboard plugged in and see if you can get it to boot consistently, if you can add a component and repeat until you find which one starts the problem, if you still cant boot then check over the motherboard connectors including all the card slots, ram slots and power connectors using a magnifying glass (a 50mm camera lens is a good substitute) looking for bent or distorted pins, cracked plastic bits etc. also check the edge connectors on the video card and the pins in the psu connectors for discolouration as that might give you a clue.

Had a very similar problem to yours on a PC many years ago. Turns out it was the processor. I had had 3 new Mobos, 2 power supplies, new RAM, graphics cards and HDDs everything. Sent the processor off to be tested and it turned out that was the problem. Might be right but as you've said these intermittent problems are a real bugger.

I've already swapped out RAM, video card and CPU with wifey's machine.

I bought all the bits for both - and they are all interchangeable (with the exception of the power supplies - as mine came already installed in the case).

The *only* bit that has to be tested in another way (but I don't know what that will be) is the mobo.

I don't have the heart, or the correct wife, to strip both machines at the same time and rebuild from zero - especially with the threat of her inheriting the faulty component.

I'll buy another mobo if I can't get them to cross ship another one on an RMA.

Everything is so *backed up* and protected that it's now more of an intriguing challenge than a disaster.

FOUND IT!!

Intermittent connection between video card and motherboard.

Bought a PCI card to try instead of the PCI-E card. Worked a treat.

Motherboard still under warranty, too. Yay!!

Six months to track the fault. Any marks for staying power?