It sounds as if ancient FCP dies not like something about the files, but you haven't told us enough about how the files were converted to QT or to which codec, and without more specifics it's tough to help.
David Roth Weiss
Director/Editor/Colorist
David Weiss Productions, Inc.
Los Angeles
http://www.drwfilms.com
POST-PRODUCTION WITHOUT THE USUAL INSANITY ™
A forum host of Creative COW's Business & Marketing and Apple Final Cut Pro forums. Formerly host of the Apple Final Cut Basics, Indie Film & Documentary, and Film History & Appreciations forums.