Sound but no pictures?

I am obviously doing something wrong....!

When I insert a title in the video, I lose the picture but retain the sound.
It is fine for the Title at the Beginning of a Movie, that works fine and
then the movie plays fine but after about 5 minutes I have put a title in and
thats where it all goes wrong! There is sound but no picture but all the
other titles are there (5 in total) so I have a blank screen with sound,
every 7 minutes or so a new title but nothing else - help! Thanks