I believe that I have a solution (though of course, seeing as the problem does not always instantaneously show up, it could come back, and if it does, I'll post).

In fairness, I'm not entirely sure what did it, but it was either running FSX as administrator (I don' think that was it), or using this website--http://www.venetubo.com/fsx.html (courtesy of LOU)-- to create an optimized version of my fsx.cfg file.