Simplenews + Mime Mail + SMTP = occasional blank email
First step. Have you combed your logs in Drupal for errors?  And have your also looked at your web server PHP error logs?

With this random problem you are having I would expect to see something going on in the logs.

I have never used the Simplenews module or reviewed it's code so I cannot say how well the authors of the module addressed error handling the module.

Clearly you have a problem and it's been reported by other people. If you find no error in the logs then I would say that potentially the module does not have enough error handling written into it to diagnose the problem.

For what it's worth (purely for your own information), I will say that attempting to do an email newsletter from a a web server is not the preferred method. An email service provider should be used for that functionality if you have any concern for deliver-ability of your newsletter.