Is it dead? (Gigabyte X99 SLI)

[MaximumTroubl]

Hello, everyone.

I am having a big problem with a computer I built after deciding to ditch AMD in favour of Intel. I got a Gigabyte X99 SLI motherboard and an i7 5820K. The seller allowed me to bring my own PSU to see the components working before buying, so I know the mobo and the CPU worked well with the PSU before this.

Once I got home, I started to assemble the whole thing. I got 4 x 4 GB AData RAM installed, but the computer would only bootloop (turned on for some seconds, then turn off, repeatedly until I turned off the PSU). Removing two RAM sticks allowed the computer to post, and then I checked the manual in order to see the correct way of installing 4 sticks. I reassembled the whole thing again, but now there was no post, just fans spinning. I removed one RAM stick and it posted, but trying to access the BIOS resulted in a black screen. I tried to see whether the RAM stick I removed was faulty, so I inserted it in again. The bootlooping problem returned. At that moment I realised it might had been a RAM compatibility issue, so I thought of updating the BIOS. I switched the PSU off and removed the problematic RAM stick so it could at least post. However... now it does nothing. No fans, no power, nada. I checked all connections and cables, but it does nothing. I tried the PSU with an old Pentium 4 motherboard I have lying around, and at least the fans spin. I don't really want to think I killed the whole mobo+CPU pack, since it cost me quite a lot.
The PSU previously worked well with my previous setup (which included an AMD FX-8370).

Parts:
Motherboard: Gigabyte X99 SLI (2011-3)
CPU: Intel Core i7-5820K
RAM: 4 x 4 GB AData DDR4
GPU: AMD Radeon 7850
PSU: Cougar STX550 550W 80 Plus
Storage: 4 assorted hard disks, 1 BD drive, and 1 SSD. All were disconnected when dealing with this problem.

TL;DR: After battling with (alleged?) incompatible RAM and bootloops, the mobo does nothing, but the PSU seems to be OK. No fans, no beeps, nothing. It worked well before.

Any help? This is so frustrating... Thank you very much...

 

[Tumeden]

Try one stick of ram.

 

[atomicWAR]

It is possible your PSU is to blame. It is not a horrible one but its not a great one either. How old is the PSU? Also have your tried a bare bones build outside of your case?

 

[MaximumTroubl]

Thank you for the quick answers, guys!
I have even tried without any RAM, but nothing happens. The PSU is from 2014. I will check my flatmate's PSU, I might try with it.
Before going to work, I put everything on a glass table so I could do a barebones build. I didn't have much time, but it did nothing anyway. I will try again when I get home.

 

[MaximumTroubl]

I finally got it solved, though I don't know how.
I tried to use my flatmate's PSU to no avail (it's a Corsair 460 W). Then I was about to give up, when I tried one last time with my own PSU. Fans started to spin again. Then I discovered that one RAM module seems to be faulty (that was the reason for the bootloop). Now I got my system up and running, though there were some weird things.

First, Windows setup was presented in 640x480 grayscale resolution, and it even started that way until I installed AMD Radeon drivers. Second, I can't enter the UEFI settings without pulling out the mobo battery to clear settings first; the system does not respond to ESC or DEL during post (it boots to Windows anyway). I suspect the reason for all of this is that my Radeon 7850 isn't really UEFI-compatible, though it worked well with my previous UEFI-enabled AMD motherboard (with an UEFI-aware Windows 10 install).

 

[MaximumTroubl]

RAM sticks were improperly seated, that was the reason the computer didn't post. One RAM stick was faulty.
The greyscale Windows setup is because my card isn't UEFI-compatible.
The system would not sleep because I had installed a beta SATA/AHCI driver from Intel.