I suggest taking a recent backup copy of your file (one that hasn't crashed) and running a recover on it to see if any problems are reported. Given that you have FileMaker 9, you might even download a 30 day trial of Filemaker 11 and try using it as they upgraded the recover tool, but the file format for version 11 is the same as 9 (and the same all the way back to version 7).
You might find that even your back up copy that you are using has problem. Importing into a clone rebuilds all the indexes so this may fix the problem temporarily until the hiddent damage is somehow encountered and it then munges a key field index--resulting in your crash. This is strictly speculation on my part and I've never seen the internal source code for FileMaker so I may be wrong in this theory, but there's no harm in running a recover to see if there's a hidden problem or not. You can also Use FMDIff, a third party program to check for damage to your file.
Things to keep in mind about Recover:
- Recover does not detect all problems
- Recover doesn't always fix all problems correctly
- Best Practice is to never put a recovered copy back into regular use or development. Instead, replace the damaged file with an undamaged back up copy if this is at all possible. You may have to save a clone of the back up copy and import all data from your recovered copy to get a working copy with the most up to date information possible.