Page goes missing!

I am having an on-going problem with a newsletter I am writing using Pages 09 on an imac running 10.6.8. This newsletter contains text and pictures, and is four pages long. When I update it for the new month's issue, I start with the current issue, delete old text and pictures , and insert new text and pictures. This works fine most of the time, but sometimes when I delete a picture on, say, page 2, the last page, page 4, and its contents disappear, and I am left with only pgs 1,2,and 3. If at that time, I do a "Edit / Undo" the fourth page mysteriously reappears with all content intact. Can anyone explain what's happening, and what to do to avoid this problem?

Thanks,

Kevin

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