FTP larger files is blocked?

I am trying to FTP images to my web server. Lately, any file that is larger than 10kb gets stopped at 10kb, and the server times out. This happens on a Win98 machine with SmartFTP or WS-FTP-LE and it happens on a XP Home machine with SmartFTP... (at 8kb). It does not happen on a Win 2k machine with Reflection FTP. All 3 machines are on a switched LAN with DSL upstream. If I go to the settings and change the Speed Limits, I can sometimes get farther and sometimes send larger files... this is if I limit the speed to 2kb or so. Much higher, and it freezes at 8kb. Any ideas?