In this case you should try testing the system using a direct connection between the Tablet PC and the external monitor connected and do not test using a dock station. Also test using the latest generic driver on our site, available here:
If the issue persists, try using the cusotmized driver provided by your computer manufacturer.
Keep in mind that the generic driver on our site may not be optimized to work using a docked connection since these are options that are implemented by each Original Equipment Manufacturer, in which case they validate their drivers to support such customization. Therefore, you may experience that kind of behavior when using our generic driver