Could you make sure your site's memory is not limited? It often causes symptoms like the ones you're experiencing:
http://codex.wordpress.org/Common_WordPress_Errors#Allowed_memory_size_exhausted
If that doesn't help, could you check your server error logs around the time when you experience these "white screens of death" as we call them? It might tell us more about the errors and what's causing them.
If you don't know how to access your error logs, your hosting provider should be able to help.
Thanks!