Can't upload

I can send text emails but whenever I try to email an attachment, my modem disconnects or stops responding and freezes. The upload will start but will cease after a few seconds or a minute. Downloading is okay. This happens regardless of email application, i.e., OE or Thunderbird. It also happens with other applications such as file-sharing apps. My antivirus is up-to-date. Has my modem gone bad or is some setting wrong or what? I can't understand why this happens.