1) Always click the DONE button when you finish editing a title. If you are already doing this, read on.
2) At 50 minutes, your project may be too long for iMovie. I would try splitting it into two parts. You can do this by selecting the project in the Project Library List and clicking FILE/DUPLICATE PROJECT. Do this twice, and name the first one "your project name part 1" and name the second one "your project name part 2".
Open Part 1 for editing and delete everything in the second half. Open Part 2 for editing and delete everything in the first half.