I'm guessing, but it may be because Miami is receiving too many netbios packets and is suspending smbd for a while. Try going to the Miami database in prefs, go to SysCtl, and set inetd.toomany to 50. Enabling logging in samba and in Miami will also help you track down the problem if this doesn't work.
Samba does quit transfers sometimes even then, seems the Amiga port is not too stable. But you should be ok in most situations.
