I'd run them for at least four full passes. If you get an error that points to a memory failure, swap your memory modules' slots and run the test again. If the error moves, then you know which module to replace. Or you could try upping the RAM voltage a 0.05V at a time to see if that clears it up. Retesting after each step.
. Sometimes just reseating the modules will clear an error.
.bh.