Make a bootable memtest86+
media (USB stick or CD) and then boot to memtest86+ after POST. Let memtest86 complete its cycle with no errors before you test the next stick; it could take an hour or more depending on your RAM.
The bootable compilations are about 1/2 way down the webpage.
You should test each stick of RAM separately to see if there are errors. And then run memtest86+ on all the sticks together.
If the RAM passes, you have another problem.