How do I prevent a page break when the detail lines are forcing it and also the Group Header section?  I only have 'New Page After' selected in Group Header section but when the report comes to the end of a Group and the detail lines are at or near the max it forces a blank page.

no, just in the goup section.

My guess is the 'Keep together' Option in the section expert. A common misconception of the 'Keep together' is that it is in reference to multiple sections. It is only in referecen to that one section. So if your group header section is set to 'keep together' it will force a page break if it cannot print the entire group header on the one page (it is not referencing the entire group, e.g. header all the details and the footer).