Index bookmarks not appearing in correct place in PDF

This has happened to me on Frame 6, Frame 7, Frame 8, and Frame 9. It's happened with Acrobat 7, 9, and 10.

Occasionally, when I print a book in FrameMaker, the Index bookmarks are out of sequence in the generated PDF. They appear as subheadings in the file BEFORE the index, and the Index bookmark itself has no subheadings.

For example, the bookmarks look like this:

Title

Table of Contents

1 Introduction

2 Basics

(other chapters)

   Symbols

    Numerics

    A

    B

    ...

    Z

Index

In this book, the bookmarks from "Symbols" through "Z" are in the Index file. But, they don't appear in the correct sequence in the PDF.

Note: To create PDFs, I use the Acrobat PDF printer driver, distilling the book to a ps file, then running Distiller to create the PDF. When this problem arises, I move the bookmarks to where they should go

This is a great puzzlement. Can anyone tell me why this happens?