Why not take them to a related list view instead? If they're doing that much "work" in the portal, it might make more sense to give them a fully-fledged list view.
Also, you may want to have "always display scroll bar" set up, so they can click the empty space to page up or down on their own.
Lastly, you can script portal navigation using the "Go To Portal Row" script step. It supports first, last, next, previous and a number by calculation.
The Get(ActivePortalRowNumber) function will also come in handy if you decide to script Go To Portal Row by calculation.