White screen flashes every milliseconds on iMac - After sleep mode

When my iMac goes in sleep sometimes and I log on again and works normally, the screen flashes white and comes again many times after awhile. I updated the all applications and I still get this issue. This started happening from like 3 or weeks ago; every time I had to restart the computer. I face this issue usually while I am preparing emails or Instant Message on Outlook and Skype for Business.

This week is fine, but it may happen again.