Hi KJB11,

I understand you are experiencing a flashing screen on your MacBook Air display.  When did this start happening?
I'm curious if the screen flash you are experiencing is the same one that is used in the Accessibility pane in System Preferences.
Try this and let us know if this is the screen flash you are seeing:

Click System Preferences > Accessibility > Audio and click on the Test Screen Flash button.

KGB11, try restarting your computer in safe mode: OS X El Capitan: Start up in safe mode  Does the issue persist?

Try also resetting the SMC and NVRAM on your MacBook Air:

Reset the System Management Controller (SMC) on your Mac
How to Reset NVRAM on your Mac

If the display continues to flash after trying these steps, you may need to have your MacBook Air inspected by a Genius in an Apple Retail Store or by an Apple Authorized Service Provider.  Click on the links below to initiate that support:

Take care.