Are you editing with your timeline set to an edit friendly codec?
If you are trying to edit H.264 material, Final Cut Pro 7 will exhibit all kinds of weird and unpredictable behavior.
FCP7 only gives the appearance of working correctly with the format, there may be unexpected consequences (such as during export, playback or rendering) where problems will appear.
(This is because FCP7 was abandoned by Apple before h.264 became the ubiquitous format it is today, and was never made capable of reliably editing in it.)
The best practices workflow for dealing with any source format that is not listed in the Easy Setup menu is to transcode the source material to an edit friendly format prior to ingest (ProRes) and then import the files and work with them in FCP.
If you've already invested a lot of effort into your edit then you can continue to edit with extreme caution, or switch the codec of the timeline to ProRes, which will require you to render more, but will give you more stable performance and predictability.
MtD