Server timeout when uploading

On one of my websites, I can log in ok, but whenever I try to upload a file, the transfer always freezes almost immediately, and I then get a server timeout message. The only files which I can successfully upload are those which are zero bytes in size.

Although I have a cable modem connection, to upload to the website in question I have to dial to a particular ISP account using a modem. This used to work fine, but I got a new PC recently and then the problems started, so maybe it is getting confused about me have two simultaneous internet connections.

The problem is not restricted to SmartFTP - it happens with all FTP programs. Has anyone seen anything like this before? My ISP's tech support has so far been unable to help.