Don't know how new you are to FCP so have a look first at one of Shanes Stock Answers
Shane's Stock Answer #12: Dropped frames on capture/playback
http://docs.info.apple.com/article.html?artnum=58640
1) Do not capture to your main system drive. Since it is busy reading the operating system and application files, it will intermittently drop frames during capture. Capture to a separate internal drive, or external hard drive (firewire and eSATA for example).
2) Deactivate any anti-virus/filesaver software, including Norton and Virex. For some reason these programs think that the large files created when you capture media are in fact caused by some sort of virus, and they try to prevent this.
3) Check the format of the drive you are capturing to. It should be Mac OS Extended, journaling off. If it isn't, copy your files from it and re-initialize it. If it is any other format, you will encounter problems. If not at first, then eventually.
4) Trash the FCP preference files. Use FCP Rescue available here:
http://www.fcprescue.com
5) Make sure that the hard drives you are capturing to are fast enough to handle the footage being captured to it. A regular firewire 400 drive cannot capture uncompressed HD, or even uncompressed standard definition. A RAID array of drives might be in order for these formats.