This is almost certainly a setup issue between your sequence and FCP 7. See this blog post for an explanation and a solution.
To check whether your setup is OK, follow these steps:
- Download the CheckFPS utility.
- Open your project in Final Cut Pro.
- Click on a blank area in the Browser so nothing is selected.
- Choose the menu item File > Export XML...
- Start the CheckFPS utility and open the XML file that was exported in step #4.