Model : vostro 3558
Thanks for posting. Apologies that your system is not performing as expected.
Flashing screens are sometimes difficult to diagnose as the problem could be from either hardware or software.
Have you tried an external monitor? Do you still see the flashing there?
Does the screen do it all the time like when using email, internet, or productivity software?
Here is information you may find helpful: Troubleshooting a Flickering Display after a Windows 10 Upgrade