Hi skurken
Sorry to take so long replying, I missed your post somehow yesterday
I'm afraid I've come to the same conclusion as you. As you've replaced nearly everything that could have been causing the problem (memory, PSU, GFX) the only thing it can be is the mobo.
It may be worth checking the slots that the memory plugs into for damage and perhaps even cleaning them out with a paintbrush and vacuuam cleaner just in case something is causing a short inside.
I sometimes find that cleaning the memory contacts with a plastic eraser sometimes (rarely) helps with random memory faults.
Hope this helps