If you run startx every time but the problem only happens sometimes, that is weird. The quickest workaround would be to run the following instead of just startx:
That will start a bash login shell after you exit fluxbox. The downside is that if you're not shutting down the box when you leave fluxbox, you'll need to exit that shell when you logout.
Have you made any changes to startx or is it the default version that comes with Slackware?