looks like marble madness! i also thought about doing a project like this because it could be done very nicely with the physics engine...

do you apply a force or torque to the ball?
did you try to make the level geometry thicker?

code:

---

         |
         |  block2
        *|
--------++--------
        |
 block1 |
        |

---

for example block1 should be very thick and block2 should be expanded further down.

* is the ball

...
i noticed the second problem in my experiments too, but it didn't bother me much because if the ground has a grass texture for example, it could be a little stone or something which causes the change of direction!