Here's how to do it:
Click Start the button.
Select Settings.
Click Control Panel.
Double-click on the Modems icon.
Select your modem.
Click the Properties button.
Click the Connections tab.
Click the Advanced button.
In the "Extra settings" field, type S36=7
Click OK to save your settings.
This will force your modem to try to stay connected at high speeds in two different ways before dropping back to an asynchronous mode with auto speed buffering.