Can only boot with 1 stick of Ram.

Morning you lovely people, hoping you can help me with a little problem I'm having.

Basically, the title says it all. My rig has started powering itself down on POST with more than 1 stick of Ram installed. Occasionally it passes POST, at which point my BIOS comes up (well, just the "Gigabyte UEFI" bit) then it shuts itself down, and tries to restart again a few minutes later.

Now, I've already resigned myself to a hardware fault, and something needs to be replaced, but I am on a tight budget, and need to make sure that I replace the right thing, as I can't keep throwing money at it.

Things I have tried:

1) Tested all 4 sticks individually in all 4 slots (all were fine).
2) Tried different combinations of slots/sticks (basically, if more than one stick goes in, it doesn't work).
3) Cleared the CMOS

A quick google search tells me that it's probably:

1) A bent pin on the CPU's memory controller. (Seems unlikely, I haven't touched my CPU for over a year, and this only started yesterday)
2) A faulty Motherboard
3) The power supply - I'm not sure about this one, the PSU is pretty old (2-3 years) but it was a good one (OC Gold 80 I believe). It was a brand name anyway.

Anyway, any ideas? As I said I don't mind replacing hardware, but it needs to be the right piece.

I have tried pretty much every combination i could think of, slot 1+2 (single channel ), 1+3 (dual channel) 2+4 (dual) and 4+3 (single).

The problem has now deteriorated, and the machine won't post at all, even with one stick, so I can't check the bios. Which makes diagnosis a bit more difficult

Hmm ok that's interesting. Removed everything except the keyboard and the CPU and I got a continuous short beep - which according to my manual means power error.

Reconnecting the memory goes back to no beeps, just a power down.

Connecting the GPU only continues with the power error beep.

So, that sounds like it might be the PSU? - oh I hope that's the case. I have a 5 year warranty

I agree with checking power supply. Do you have a multimeter?

Another thing to try, if you haven't already, is to disconnect all of the power connectors from the power supply to all of the components. Carefully inspect all connectors for dust, bent pins, or any signs of scorching. Clean all connectors and sockets with compressed air. Check the RAM slots at the same time while you have the compressed air out - a single piece of hair can gum up the works. Then start adding components back one by one again.

Often when a PC won't boot when adding a know good stick of RAM it is because of a voltage problem. The power supply is a good suspect, but it could also be a problem on the motherboard. You could go into the UEFI and check the board voltage readings for anything obvious.
You could also experiment on the Advance Tab with increasing the RAM voltage in 1/10 volt increments just to see if you could get a second stick to boot. I would not suggest going beyond 4/10 - 5/10 higher however.

I've started the rma process for the PSU just in case, I have a friend who is going to supply a spare PSU in the meantime so I can test. Will let you know, cheers for the help so far. As I said, I can't actually get to the uefi as it powers down before the gpu turns on, which makes me think it probably is the PSU, maybe the 4 sticks were sending it over the edge

That's a good plan.

Just for information: you can boot with just the one stick and make the voltage tweaks, then add a second stick and see what happens.

750W cooler master supply should be plenty - nowhere near 'the edge'. But it could be just defective.

So it turns out it was the motherboard. I borrowed a new PSU from my friend, nothing changed. I've just purchased a new motherboard and everything is good.

Well almost anyway. Turns out that LGA 1155 motherboards are rare as hell. Ones with 4 DIMM slots are non existent. So I had to buy a mATX motherboard with 2 DIMMS, fight it to get the graphics card and the SATA III port connected at the same time... and crucial no longer sell 1333Mhz ram. That's a story for another thread though.

Thank you for all your help.