Collision is notoriously difficult.
Java3D doesn't make it any easier really.
There are many, many factors that contribute to final accuracy and performance, including the testing geometry as well as the scene organization and shape SIZE and triangle COUNT.
Here a couple links to review that may help, but just keep in mind, "simple" navigation with collision is NOT simple, so don't get discouraged!
Implementing Terrain Following and Collision Detection in Java 3Dhttp://www.j3d.org/tutorials/collision/index.html
Practical Collision Detectionhttp://www.gamedev.net/reference/articles/article736.asp
General Collision Detection for Games Using Ellipsoids http://www.gamedev.net/reference/articles/article1026.asp
Scroll down to Collision Detectionhttp://www.gamedev.net/reference/list.asp?categoryid=45#99
Some Math and Physics for good measure!http://www.gamedev.net/reference/list.asp?categoryid=28