For problem two it may just be because if you used an effect. Sometimes when using effects it will make the screen go black or completely stop. When I used WMM when I used the speed up effect sometimes it would speed to the end of the vid. ^__^'
Another annswer to that problem is you may have a scene sliced too small. Most times it will play, but there are those instances where small sliced scenes just don't play. This has also happened to me and my friend <a href="http://www.animemusicvideos.org/members/members_myprofile.php?user_id=719435">SageKiller</a> before too.
Life is filled with good memories and bad ones, but memories are what you make of them.