Sympathize greatly... you are between a rock and a hard place.

My dad is mid-eighties and drives okay, but when he has a car prob, one
of us sons has to drive it to determine what it is... and dad used to
know mechs pretty well.

If there are no codes to read out (has the dealer tried?), I see no way
to resolve the problem other than switching cars with your mother and
seeing if it happens to you.

- like if the tach drops to zero, immed.. or does it reflect the engine
as it dies.

Here's a wild and crazy idea. Try a different garage.
I'm not even saying the first garage is bad, but a fresh look might
help. Ask to have Ford technical support get involved. There are a few
electrical components that are most suspect in these intermittent
cases. If it means enough, they can be changed out.
I remember in the old days with the first Ford modules, people could go
past certain high voltage lines or other interference and the engines
would stall out. They would sometimes restart when they coasted past
the critical spot. GM modules did not do this because they were buried
in the distributor. I recently ran into a Blazer transmission control
module that has malfunctined twice in exactly the same 100 M stretch of
road. I don't know if it's just coincidence or deja vu all over again.

Have you replaced the fuel filter? This will cause this symptom... ?
Coil connections? Possible ingition switch. The fuel filter was an idea
until I read that it happened 5 or 6 time since owning the car, how
long has she owned the car. A long time sounds like Electrical to me.
Or computer, has the battery been disconnected latley? Just some
ideas...  

Gerald