FileMaker does not save with every keystroke--as you have discovered. It does save periodically and this setting can be modified. Check out the memory tab in Preferences.
First thing I'd check is to make sure that the file is not damaged. Not only might file corruption explain the crashes, the crashes themselves could damage your file.
Launch FileMaker without opening your file and do a recover on it to see if if finds and reports any problems. Even if you do not get a report of any problems, test the recovered copy to see if it shows the same instability. (Best Practice is to not actually use a recovered file, but to replace it with an undamaged back up copy if you have one. You may need to open the back up, save a copy with the clone option and then import all your data from the recovered copy into back up copy to get the most up to date data possible into your back up.)
If that doesn't help, test your system by creating a new test file from scratch with just a few fields in it and test it to see if it shows any problems. If you get the same behavior. If this test file is unstable, then you may have a deeper problem. 10.6 has had several issues with fonts, so you might research that possibility here and at Apple's site. You might delete the preferences file for FileMaker and see if that helps. You could uninstall and re-install, but I wouldn't try that until these other options were explored.