Motherboard / CPU/ PSU not powering up...

[max_allan]

Hello,
I'm really struggling here. I've built plenty of PCs before and never had a smidgen of problem.

So I have a new LGA 1155 mobo and I5 CPU.
Disconnect the PSU (700W rated) from the existing working mobo/GFX/CPU and plug in new mobo/CPU. Green LED comes on. Hit the power switch and the fan sort of quivers a bit, sometimes it runs and sometimes the speaker makes a bit of faint croaking noise. No graphics output and no post beeps.

Assume it's a bad mobo and send it back. Get it tested and returned no fault found. Cost £££.
Feel annoyed and decide to get a cheap mobo/CPU off eBay so I can do my own testing.
This is supplied with CPU fitted and supposedly tested before shipping.
So, plug in 2nd 1155 mobo and celeron.
Pretty much the same thing happens. No noise from speaker and no reliable fan motion. No GFX output.

About this time I think it might be a bad PSU so I plug the old mobo/CPU/GFX card (think it's LGA755 and pentium Q6600 quad core with a Nvidia 8800 GTX - it's going to eat power compared to a little celeron)
Everything powers up, get POST output on GFX card.

Slightly confused now. So try another PSU (300w generic) and exactly the same problem as with the other PSU.

So I have 2 PSUs that will power up other machines that don't work on LGA1155 motherboards. The only element I haven't tested by now is the I5 CPU from the original rig.

I have tried plugging in all 24 pins or just 20 from the PSU and using or not using the 4/8 pin "CPU" connector (I only tried without the 4 pin connector once just in case! Didn't expect it to work.)
I am using the same "power" switch and speaker on the working board and the failing ones. It does beep on the good mobo because of no boot device.

I have tried it with and without RAM.

Anything I'm missing?
Do LGA1155 mobos take more power on certain pins or need a PSU that is less than about 3 years old or something weird like that?

Really starting to pull my hair out!

 

[max_allan]

OK, slight update. I have measured the voltage on the 4pin 12V "CPU" connector. On the generic 300w psu they are generally stable at 12v. On the quality 700w (It's an FSP Epsilon 700w) the voltage floats around for about 5-10 seconds and eventually settles quite high about 12.5V Sits there for a bit and then the PSU seems to turn itself off.
It's like it's being shorted out, except the PSU doesn't start boosting it's fan speeds.

So I found a 3rd ATX PSU. It's a 350w or so HP PSU and it does the same thing.

And I'm running the boards on a range of non-conductive surfaces, foam, card, plastic - all surfaces that I have run the old working board on.
And I have checked the VGA cable and monitor with a laptop.

 

[max_allan]

Anyone???
2 days and not even a "you've done everything right, I'm confused like you" or "you're doing everything wrong idiot". I'm really thinking this is a PSU issue but as I already have 3 that work on different boards, I really don't know if I want to buy a new one.