Thank you for your post.
I am running FileMaker Pro 8.5v2 on an iMac with OS X 10.5.6 and not encountering this problem. Let's try a few things.
Create a new database file with one table and one field. Enter a few records and sort. Does this crash? If not, then perform a find and sort. Does this crash?
If either method crashes, then something within the application is causing the crash. Uninstall and reinstall FileMaker Pro.
If the smaller works, but only the one file is having a problem, then it might be a corrupt file. Close the file, pull down the File menu and select "Recover..." Select the file in question and let it default to <filename> Recovered.fp7. When finished, open the Recovered file and try the same steps to make it crash. Does this work?
Let me know how this works before I know which further path I need to take.