642-831 Problems

It is evil. I scraped a pass 6 months ago but it has been suggested that they have recently been revised and made more difficult.

I don't think you'll find any practice tests or books that help you out much more than you have seen already. The best thing is probably to get some real kit and start using it, I did that after my CCNP and it's helping a lot with my CCSP, wish I'd bought it earlier. The other option is to try one of the sims like the Boson ones or the Mimic labs. I think Boson's latest sim is currently available as a free download (beta).