If your modem it timing out during file transfers or loading Web Pages,
you might try increasing the timeout period.

To change it:

Start Regedit
Go to HKEY_LOCAL_MACHINE / System / CurrentControlSet / Services / Class / Modem / XXXX / Settings
Where XXXX is the number of your modem
Move to the right panel and double click on Inactivity Timeout
The number of minutes for a timeout should be entered between the brackets.
For example, a US Robotics Sportster could have S19=<30> to set it to 30 minutes.