Make sure that any loops have some way to escape. Make sure any find commands are handled by your script and not left to filemaker. For me those have been the biggest problems with scripts running. IMHO that fact that it runs ok and then breaks seems to point to data changing that can trigger one of these areas.
Thanks for the advice. I'll look into it. This script stack is inherited from a previous employee, so I'm not clear about all the loops yet.