After you attempt to fix your enconding issues (see my reply to your other post), I would suggest that you have a look at the apache error log in the server, if you have access to it. Or ask your sys admin to have a look for you when that blank page happens.
You could also use the tiki-check.php script to find out potential problems in your setup. Blank pages usually come from low memory allocated to php processes. But they could come from other places, depending on cases.
You might also benefit from enabling the Debug mode in your tiki, so that you configure tiki to allow displaying error messages on the screen for you, so that you have a clear idea of the source of the problem, etc. See:
http://profiles.tiki.org for profiles in general, and this is the one I recommend: