Ok. Do you get two group headers - one with the data for the previous group and one with next group? If so, how large is the group footer? Is this happening with all of your groups in this report, or just one specific one?
What I think may be happening is that your group footer has white-space at the bottom, so it doesn't quite finish rendering on the previous page when your other sections are wider. So, it goes to the next page and you see the Header (because you have Repeat Header on every page turned on.) Your best option is to reduce the amount of white-space in the footer - unless I have summary data that I need to display, I usually use a fairly narrow group footer, frequently with a line in it to visually separate the two groups.
-Dell