Have you read the Physics basics tutorial?
hings like incredibly heavy objects piled on a stack of really light boxes, huge piles of objects, or very fast moving heavy objects tend to cause instability. Try to keep everything in your game at reasonable proportions.
I would expect a density ratio of 100:1 is too much for the physics simulation to handle. You should keep things far more reasonable otherwise you can't expect realistic results - I'd say 10:1 is more reasonable for densities.