I don't know whether this will help but it's always worth doing before delving deeper.
Have you tried trashing the preferences?
Many weird things happen as a result of corrupt preferences which can create a vast range of different symptoms, so whenever FCP 7 stops working properly in any way, trashing the preferences should be the first thing you do using this free app.
Shut down FCP 7, open PreferenceManager and in the window that appears:-
1. Ensure that only FCP 7 is selected.
2. Click Trash
The job is done instantly and you can re-open FCP 7.
There is absolutely no danger in trashing preferences and you can do it as often as you like.
The preferences are kept separately from FCP 7 and if there aren't any when FCP 7 opens it automatically creates new ones . . . instantly.
APOLOGY: I was in the wrong forum.
Message was edited by: Ian R. Brown
We need more information about your video:
Where is the video (and render files) stored? If it's on the boot drive, that could very well be contributing to the problem.
What format is the video you're editing? Formats which don't work natively in FCP, like H.264, can cause a lot of issues that you're describing.
Does the video match your Sequence settings in FCP? Not matching means a lot of rendering will be required for playback.
Are there any still images in your Sequence? All still images must be in RGB Color Mode. Images in CMYK, Grayscale or Indexed Color Modes can cause FCP to crash.
If you want to analyze the crash log, this app can help: http://www.digitalrebellion.com/promaintenance/