I would recommend applying the latest hotfix first to exclude already known and fixed bugs. There were some in the email engine as well as in scheduler.
Then, if you do not want to use the email queue, use appropriate overload which sends the emails immediately and not through the queue - using the "sendImmediately" parameter set to true.
For the scheduler, please read the documentation on scheduler reliability to learn how scheduler works and what can be done to achieve better reliability.
Certain tasks can be executed in the live site app as well.