In XAML, I have set up a Grid, containing a ScrollViewer, containing a TextBox:
Title="MainWindow" Height="350" Width="525" Loaded="MainWindow_OnLoaded">
<TextBox Name="TempTextBox" ScrollViewer.HorizontalScrollBarVisibility="Auto" ScrollViewer.VerticalScrollBarVisibility="Auto"></TextBox>
TextBox has scrolling support out of box, it doesn't need this addictional control 'ScrollViewer' to have scrolling. Set ScrollViewer.VerticalScrollBarVisibility="Auto" in your TextBox, but don't wrap up your TextBox with ScrollViewer and you are good to go.
You are giving two scrolling possibilities which are conflicting with each other.
I have a TextBox with ScrollViewer.VerticalScrollBarVisibility="Auto" without ScrollViewer and Page Up and Page Down works fine.
To test, I added 'ScrollViewer' around my 'TextBox', then it started to show the same problema that you have stated.