1. Try connecting an external monitor and see how it behaves. See the following link.
If you have no spare monitor go to item 2.
If you have no problems there may be something wrong with your screen.
If you still have problems there may be something wrong with your graphics adapter.
2. If you click on Toshiba Assist icon, you should hopefully find a program called PC Diagnostics.
To quote the "blurb":
''Toshiba PC Diagnostic Tool provides you with a user-friendly application that can be used for analyzing the system configuration and obtaining detailed information on each of the connected hardware components.
However, its functionality is not limited to displaying the description of each piece of hardware. The application can also be used for diagnosing devices that have problems working properly and running benchmark tests meant to assess their performance.''