Weird RAM/MB issue, computer won't boot

First, this is my computer:

MB: Gigabyte EP-45 extreme.
PSU: Antec Quattro TPQ-850w
RAM: Crucial Ballistix 2GBx8 DDR2
CPU: Intel Q9550 2.83Ghz
HDD: Seagate 7200.11RPM 300Mbps 500GBx2

The problem:

When I first bought my computer, 2GB was the standard, so I bought OCZ Reaper 1GB x 2 of ram. I installed everything and when I tried to boot the computer, it beeped (don't remember how many times, I looked into the manual and it was a ram related beep), so I took one of the sticks of and the computer booted up so I thought it was a DOA ram. That same year, later I decided to upgrade the ram since 1GB wasn't enough (I was using XP since Vista sucks and Windows 7 hadn't been released), I bought a pack of Crucial Ballistix 2GB DDR2 x 2 for a total of 4GB. I unpacked the ram and installed them, then tried to boot and again the same beep error, I removed one of the sticks and it booted. I turned the PC off and tried another slot and BOOM it booted perfectly. Days later I started to get random BSOD memory related. I ran memtest and it found errors. I moved the sticks to other lots and ran memtest and everything went perfect. Weeks later, again, random BSOD. I was tired of this and bought the same pack again because I thought the ram was faulty. Nothing, the same issue and I'm here, getting BSOD almost everyday.

The sticks are on third and fourth slots of the MB (I know, it's not on dual band, but it's one of the configuration that works, or at least boot up the comp.).

I tried adding one of the remaining ram sticks to the MB and it didn't boot.

I think it's the MB, but I don't know.

tl;dr: computer not booting if +2 sticks are installed and it gets random BSOD and crashes while using only 2 sticks.

What do you think it might be?

Thanks in advance.