Your answers sound like you were speaking from long and thoughful experience.The problem turned out to be much more mundane; an individual with a "MARC" fact where it should have been a "MARR" fact. Linda send me the GEDCOM, I loaded it into Roots Magic, and the .LST file said it didn't recognize "MARC".
When she corrected the individual, her GEDCOM worked.
I have no idea why PAF put MARC for one individual and MARR for hundreds of others, but it did.
I should have asked her to upload her GEDCOM into PAF and read the LST file, but I forgot. My logic was 1) PAF knew and loved whatever tag was causing Ancestry fits; 2) Roots Magic (or any other program save PAF) would not know and love the tag; 3) Ergo, importing the GEDCOM into anything save PAF would reveal the faulty tag, if that was the problem.
I might be wrong; it could be that if the individual is a left-handed red head or has exactly 47 life facts, or there are some other rare circumstances, PAF generates the MARC tag, and will not recognize it itself.
In any event, problem solved.
If someone new to the game reads this, when you import, say, SMITH.GED, your program will generate SMITH.LST, usually in the same directory your SMITH data base is in; the LST file has a list of problems encountered. You can then open SMITH.GED with "Notepad" and try to find the offending tag. If you have an editor like DEC's EDT, which numbers your lines for you, the task is much simpler.