What do you mean by dynamic ports? Are you saying the port changes
(generally I've only seen IP changes, i.e. for DSL or DialUp users, but
never a port change.)
If you're running the FTP server, try to disable the dynamic ports. There
really is no point in that, assuming you have set a hard-to-guess password
on any FTP accounts.
If you're the client, i'd suggest that you call the dude who owns the server
and yell "WTF?!" as loud as you can.