If you add a line of g-code to the end of the file, you must hit enter after the line or it won't show up in Mach3. Mach3 needs to see the carriage return.

Thanks!  Sheesh, I can't count how many months I haven't used the edit code function because I didn't know that.  I only had the code window freak out on me as described by the other folks here once and it went away after a reboot and dump of that code file.

Now I just have to figure out how to prevent my machine from intermittently trying to force the entire spindle through the spoilboard.  Not a Mach3 issue - I suspect hardware.