Are you using iMovie? If so, go to preferences and turn off the option for automagically making new clips at every break.
Make sure the tape itself is clean and that there's nothing interfering with capture (AppleTalk, antivirus programs, etc.)
It's always a good idea to black out tapes before using them (record over the entire blank tape with black, ie no input signal) and never use the first 1-2 minutes of tape.
Good luck.