Sounds like a bug in your mobi. That said, Kindle previewer is pretty rough and ready, and very intolerant of minor gremlins. But if works in previewer, then it's usually bug free.
Try firing it into Calibre and use the book reader for a second opinion. Or K4PC, you can change the aspect ratio of the view by resizing the window. If your page pops back into the correct format on resize and page forward & back, then you definately have a spacing issue in your file. Check for a rogue <br/> tag or similair.
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