Hum, I did what you suggested and the problem went away. The weird thing is, I can't reproduce it now when turning the clipping plane settings back to where they were. Now I don't get the glitch at any settings (Low, Medium, High). Nice frame rate, though, so I'm not complaining. Go figure.

I may have made the SpeedTreeMaxMemUsage tweak after trying this, though, so it's conceivable that there's a relation there.

I just wish I could figure out what's causing the freezing. If I could get rid of that, I would be a happy panda.