Britgirl,
Thanks for your suggestion. According to Xvfb manpage, the default screen settings are 1280x1024x8. I didn’t realize that the bit depth was significant.
Unfortunately, I have downgraded to 3.5 after spending much time trying to troubleshoot this. If I go back to 4.0 at some point in the future, I will give this a try.
I’m running X server and GNOME from the same terminal. Will be sure to continue to do so.