I'm also getting crashes on 5.0.4.0. I setup a queue and start downloading. After an hour or so I log back in and the SF window is gone. A few versions ago I used to get a popup asking if I wanted to send crash data to the mothership. Don't even get that anymore.
bwnautilus
A couple of thoughts...
1. It could be that whatever is causing the crash isn't being caught by their error handler.
2. Do you have have SF set to exit when the queue is complete?  I've noticed that if there are problems with a download, SF will give up and move on to the next item in the queue - once the last queue item is 'handled' (whether successfully or not) the program will exit instead of putting up a dialogue informing the user that there were errors.  So, even though it may not have actually downloaded everything in the queue, it technically 'finished' the queue and exited, making it appear as if it crashed when it actually didn't.  The StreamFab.log file might have the info necessary to discern if this is the cause.