If you are still having problems after choosing ftp_pasv() you should refresh the connection through ftp_close() / ftp_connect() after x files and/or y GB have been transfered.
It solved several (random) occurring problems like:
ftp_get(): Opening BINARY mode data connection
ftp_get(): Entering Passive Mode
Hint: fopen() created a file so don't forget to delete it if an error occurred ;)