Try tweaking down the trailing edge of the wing and/or the trailing horizontal edge of the tail, to counter it. Mine was diving, and tweaking these surfaces upwards made a dramatic difference.
He talks about this after the instructions: http://www.zurqui.com/crinfocus/paper/air-fly.html
Mine fly's really well, it has a lot of lift so it could almost use some more weight in the tail section.
I added some tape to the tail section to hopefully add some weight. It didn't really help and thus my reaction is to say it isn't the "best" paper airplane. I don't think there is any "Best" paper airplane.