Thank you for your post.
Unfortunately, I have not run across this scenario. If you have to quit and restart, it sounds like a possible memory corruption or conflict. What other applications are running at the time this occurs? How much memory do you have on your machine? Anything else you can provide that may help narrow down the problem?
It never hurts to recover the file and test the recovered copy to see if that fixes the problem. If so, then your file is damaged and you should replace it with an undamaged back up copy if at all possible.
There is a fair amount of font weirdness reported in Snow Leopard. There does not seem to be a single solution.
This Google hightlights some of the problems.
A font repair utility such as Font Explorer X or Font Doctor sometimes fixes the problem.
Clearing system and application font caches sometimes fixes temporarily.
An Apple Discussions about SL Font Problems
Thank you all for your responses. After looking into this a little bit more, I have gone through some of the suggestions regarding font problems with SL, and this seems to have cleared up most of my problems.