This a problem with Mac Screen Sharing, since every other VNC client out there, including Mac Screen Sharing has the same glitch. The problem appears to happen when the external monitor has a resolution that your Mac does not support by default. Here's a workaround:

1) Before you connect your external monitor, go to System Preferences > Displays and take note of the resolutions that are listed there. Find the resolution that most closely matches the default resolution of your external monitor.

2) Connect your external monitor.

3) Go back to the Displays preferences, and change the resolution to be the resolution that you found in step #1.

4) Reconnect with iTeleport.

That should fix the problem.