Help me identify what's broken in my PC!

My PC failed to boot this morning. I turned it on and it booted for two seconds before turning off. It then restarted and lasted six seconds before failing again...once again it restarted and this time it stayed on, however it didn't get past the BIOS splash screen. I tried to get into the BIOS, thinking my SSD had failed but that wouldn't work.

I took it apart, leaving just one stick of RAM installed and the keyboard connected. It followed the 2/6 seconds cycle before lasting a bit longer on the third - long enough for me to press DEL to enter the BIOS but it didn't manage to load the BIOS screen before shutting down. I timed it at 15 seconds. Now, that's all that happens...if I start it up it'll boot for 2/6/15 before failing, unless I have the gfx card connected, in which case it'll do the 2/6 part and then stay on! But, it won't get past the BIOS screen and the keyboard shortcuts don't work so can't get into the BIOS at all.

I was thinking it was the motherboard failing/failed but the gfx card part has confused me. At the moment, the PC has been on for 20 minutes and seems to be running fine, except for the fact it can't get past the BIOS!

Something you could try is resetting the bios, should be a motherboard jumper for that or even a button if your lucky.

Failing that I would have a look at the power supply. I know its not easy but maybe see if you can borrow one or even buy a dirt cheap one just for testing purposes.

Sorry, I should have said I'd already tried resetting the BIOS - I wasn't lucky enough to have a button to do it so used a screwdriver to join the two jumpers.

I'll see if I can borrow my dad's one (it was my old 430W one) to test with a reduced load.

I'm just completely confused by how it seems to be stable when the gfx card is installed but not getting past the splash screen, yet consistently fails to boot if it isn't connected!

what cpu/mainboard do you have?

CPU is an i5-3570k with no overclock
MB is a Gigabyte Z77-D3H

The graphics card is a bit odd but I would definitely try a new PSU.

Also closely look for any damaged caps on the board

Just to update this...tried a second PSU and still had the same issue.

Purchased a new motherboard and everything is working. Couldn't see any damage on the old one so not sure of the cause but I'm back up and running now so I'm happy enough!

Thats a good result. Always worth checking things like PSU etc before shelling out on new parts as its a lot easier to check with a PSU than pulling everything apart and installing it in a new board.

Glad its all sorted :)