Ken,
That is very strange. Since you said you can see changes saved, are you saying the what you type does get into the file but isn’t shown as you type?
This might be a good issue for our Live Chat (go to Help>CodeMix>Live Chat), if there is a developer available at the time, as that feature provides us with tools for examining the issue in real time.
Alternatively, would it be possible to provide us with a sample project that includes the file that is causing the problem? As you might imagine, it will be hard for us to replicate if this is only happening with one file, so a sample project would really help.
Also, do you notice if the CodeMix engine is disconnected or in error whenever this happens? Just check the icon shown in the bottom left of the workspace window. Is there an exclamation mark shown? You can also hover the mouse pointer over the icon to see what the status is.