You're using an old version of Memtest86, try this version 5.01: http://www.memtest.org/
If memtest still hangs, re-seat all the memory modules, even swap their positions and run memtest again to at least 7 passes
I tried swapping the RAM and I had no freezes anymore, but it's a bit early to call it solved.
I cannot run memtest though, it starts running but now memtest freezes after about 3 to 30 seconds.
"I cannot run memtest though, it starts running but now memtest freezes after about 3 to 30 seconds"... this is a usual indication of bad RAM, but the system can run normally, right?