Best way to simulate lots of non-linear springs?

[aspfreakout]

Hi,

I have a system with a set of rigid bodies connected by non-linear springs (something like this: http://www1.ttcn.ne.jp/a-nishi/tensegri ... ity6_2.jpg).
Now I'm simulating the springs by applying forces to the rigid bodies (applyForce(non_lin_springconstant(position_b-position_a),offset)) directly.
This seems to work when the gravity is set to zero and the springs can be quite weak.
Now if I increase the strength of the springs to counterbalance the influence of gravity (so that the structure doesn't collapse), the simulations often explodes.
I tried lowering the timestep (as low as 100microseconds).

What would be the best way (most realistic) to simulate such systems?

Thanks!