It may be a memory issue -
When I've been unable to render a large Motion project in FCP, I developed 2 workarounds.
1. Render in sections. Mark in/out on a small section of the Motion project in your FCP timeline. Opt-A to select in->out,. Render selection. Save. Mark in/out on next section. Rinse, repeat...
2. Render a movie out of Motion. Cut it over the Motion project in your timeline. That way when you need to make changes, you still have your motion project in your timeline, you can ctrl/right-click on it and Open in Editor... then render it out again and replace the rendered movie in your timeline.
make sense?
You could post your crash log and we might make guesses about other options.
Patrick