I have had no end of trouble connecting by ssh to my main Mac Pro. Whenever I would type “ls -1” in a directory containing many files, the connection would time out. This problem came and went away periodically. Owen pointed me to a sane explanation which has to do with evil firewalls. It looks like I solved my problems (for now) by typing “sudo ifconfig en0 mtu 576” in a server shell. It has nothing to do with ssh or Apple or MacOS.