I just put the textbox in the Detail section, and code in the Section_Detail_Format event.
The trouble is that it doesn't seem to do anything, the page header is always visible.
Code:
Private Sub Section_Detail_Format(Cancel As Integer, FormatCount As Integer)
If Me.txtRunSum = 1 Then
Me.Section_PageHeader.Visible = False
Else
Me.Section_PageHeader.Visible = True
End If
End Sub