New PC, won't show display randomly

Hi,

I just assembled my new PC today and here's what happened:

When I first turned it on, it didn't show display and the monitor said "no DVI signal"
So I tried some things like using a DVI > VGA converter that came with the graphic card.. re-putting the memory in, re-putting the graphic card in, making sure all connectors are correct, and all the time the same issue.

Then, since my graphic card needs 2 3-pin connectors from the power supply, I tried switching those connectors around cause it looked like one was an extension of the other one, like this:

And to my suprise cause it didn't really make sense to me, it worked after I switched them around. So I installed windows, installed some other things and about 2 hours later, I shut the computer down, turned it back on and to my disappointment it had the same issue again.
Now the whole night I've been asking friends, looking online to see if I can find something but i'm totally clueless here, up until now it has been randomly switching from working to not working, it's incredibly frustrating.

Everything else (as far as I can tell since i can't see if it boots when having no display and i don't have a speaker for the mainboard) seems to be working when i boot it up.
The only weird thing is that the CPU fan seems to have trouble starting up when i boot the PC, it does like 2 ticks trying to spin but it doesn't, and then it suddenly starts spinning up to like 3k rpm and then slows down again. The graphic card's fans kind of do it too but only like 1 tick. The chassis fans work perfectly fine.

Here's my specs:
mainboard: ASUS H97-PRO
cpu: Intel i5-4690 3.5 ghz
graphic card: sapphire radeon R9 270X Dual-X 4 GB
memory: Corsair vengeance 1600 mhz 2x4 GB
hdd: Seagate SSHD 1 TB sata-III
power supply: Cooler Master GM550W 80plus bronze

I'm really hoping somebody has an idea what's going on.. thanks in advance.

 

You're right, i meant 6 pin. I have tried unplugging the HDD and it was the same problem, just now the problem kept persisting and i took out the graphic card, then it immediately worked on onboard DVI.
But i still don't know what is the problem.. i guess i'll have to keep trying on onboard DVI to see if issue occurs then too, because it might just randomly work now as it did before.

And what do you mean by mounting problem?