You can try the following:
1.Uninstall the current graphics drivers (Nvidia drivers) and install the latest ones.
2.Try playing games with the adapter plugged in and disable any (Power Saver, Energy efficient, Cool and Quite or Low profile modes). These modes may underclock the laptop CPU/GPU speeds and that is where the frame rate dips occur. Also, check the BIOS to ensure discrete GPU is selected. If required enable the discrete GPU permanently in your utility settings.
3. If the above two does not work or if you are unsure about checking the power settings/ BIOS settings, report the issue to Alienware customer service and have them the issue described.