Multiple records per page merge is buggy. One of the biggest is the preview button. If you've been using the preview button, most likely the template is hosed.
Try creating a new template and run the merge without the preview, or Undo immediately after using it. You can usually copy the frames from the old template without a problem, rather than having to make them again.
Thank you--it's nice to know I'm not the only one seeing this problem. Creating a new template didn't work for me, but I added more sample data and it seems as if ID is just putting the page break in the wrong place. So in my very particular case, since my data set isn't enormous, I'm going to make every 4th record blank. Looking back through posts, Data Merge has been buggy for a long, long time. I'm pretty new to Adobe software, but this is not the first time I've seen old threads mention problems that have not been addressed over several versions. Does Adobe ignore ID because their other products have more competition? That would be a shame, because it really is the best at what it does. Usually.
You might try fiddling with the margins, too.