Object behave like they are made of rubber

I am very new to PhysX and try a rather simple thing: A stack of 10 Boxes size 0.75/0.1/0.2
In the Moment the simulation begins, all boxes are pressed together as if they where made of rubber and then the whole stack is falling to a side.
The objects are created with the PxCreateDynamic function. The density is 520 and the material has the parametes 0.5 (static) 0.4 (dynamic) 0.05 (restitution).

The boxes stack is based on a bigger box created with PxCreateStatic using the same material.

Can anybody tell me what parameter I might be missing?

I recorded the whole action with the PhsX Visual Debugger. If it is helpful, the whole simulation is here:http://1drv.ms/1EI7hVL

I found a solution that seems to work:

I increased the number of calls to the simulate() function. Instead of once per 1/60 second I call it now once per 1/600 seconds. That makes the boxes stack much more stable.