chucky9 :
https://www.sony.co.uk/support/en/product/KDL-40L4000
You can run
dxdiag and provide us the info on the System and Display tabs if want to give us as much info as possible. You might also consider checking for updated drivers for your graphics card in the mean time. Lastly, how are you connecting your PC to your monitor (HDMI, DVI, VGA, etc)?