Change the sequence setting: Compressor to ProRes, and re-render.
And never ever never never use H.264 in FCP. FCP doesn't like it...you will lose sync, or exports will show different footage than what is in the cut...any number of potential issues. It is a highly compressed format not ideal for editing. H.264 and FCP are not friends.
ALWAYS convert H.264 to ProRes before editing. Yes, the files sizes will increase by quite a bit.
If you want a friend for H.264 native...use Adobe Premiere. But be sure to get lots of RAM and a high end graphics card that enables the Mercury Engine so that Adobe Premiere can deal with it. FCP solves the H.264 issue by making you convert it to a workable format. Adobe makes you throw resources at the issue. Although those resources do a good job.
What are your sequence settings?
Apple does not recommend using H264 files in FCP7. We always convert all H264 to ProRes.
You can edit with H264 but there are more posts about problems with editing H264 in FCP7 than not.
If the footage in your sequence is H264 I would consider converting the H264(at least the problem shots) to a flavor of ProRes. I would personally use PR422.