have you checked imovie preferences?
have u checked he imovie preferences? under the import tab, try disabling the "start a new clip at each scene break" option and make sure the limit scene length to X minutes is disabled.
if that fails check your recording, see whether there are "timecode breaks" in between clips. since you say it has ben happening every 5 seconds, try and pop in a tape head cleaner, as sometimes the timecode reader in the camera is faulty. is there a way to try your camera or someone else's machine? try it and seee whether problems follows you, if it does than the problem is in the camera and a good and through cleaning would solve the problem.
else the mac andyour cam ain't communicating properly