Sorry, I'm away on extended vacation in a remote location with limited network access. Which posting above did you use the settings from? Post #5 or #7?
I'm not sure if it really matters, I don't see that you're doing anything incorrectly and don't understand why Samba is crashing.
Just to confirm, are you running the x64 version of FreeNAS?
I have seen people with hardware problems (bad or failing disks) have problems with slow transfer speeds. I'm not sure what else to suggest right now. Maybe one of the other guys here has some ideas.