If you use the AwayStats (or Mr. Doob's stats) you will be able to monitor memory use in your app. Your memory is probably rising on the creation of each object until it reaches a critical state and crashes your app. You need to properly release the memory used for an object when you remove it. This could be probably done by properly destroying the removed objects and removing any reference that exists to them. Only in this case will Flash's garbage collector properly renew the memory used for this object. I'm sure someone could be more precise about this topic and explain it better to you, but I'm pretty sure you are having a memory management issue.
It might also be a good idea to reuse objects and have a limit of objects that can appear at the same time. For example, suppose you choose a max. of 100 objects that can appear at the same time. You initially create these 100 objects and then select which come in and which go out. When an object goes out of the scene, you have it available for reuse and no one needs to know it is the same object. AwayStats will help you see if you solved your problem or not. hth!
