Emails arrive blank with just header visible

I use Mac mail and recently, when I click on an email to read from a sender in the message window, the screen is black, then after a while it goes white. The words of the email don't appear until I double-click on the email to open it independently from the message window. Then, usually, when I return to the message window, all the messages appear normally.

Does anyone have a remedy?

Thx