Good preventative maintenance item, replace the tstat with an OEM
version.  I've had aftermarket tstats fail in a short time (couple of
years whereas the OEMS go for more years).

On some (most?) Fords, the fan switch is a simple temperature actuated
open/close switch that should close around 220 degrees F, located near
the tstat.  Signal then passes through the fan switch and probably
energizes a relay coil that pulls the contacts in to energize the fan.
I've had these get flaky after a long time as well.

A/C and defrost have another path from the A/C module or computer to
energize the fan, bypassing the relay.  In the meantime, just tell the
wife to leave the defrost on, this should keep the fan running
constantly if everything else is in order.  You can get by this way
with a bad tstat and fan switch.

Anyway, my point of reference is only having worked on one Ford that
exhibited similar problems, severe temp spike at idle.  Some Fords
might be different then the way mine was setup, but can't be that much
different.

if it overheats its a thermostat.
It could also be a temperature sensor  which provides a faulty reading,
and the computer doesn't turn on the FAN.

I don't know about 3.8,  but on 4.6 there are two temperature sensors,
one is for gauge,  and the other is for computer which controlls the
FAN.   They can be tested for a resistance.

On 2 occasions, my wife reported a massive cloud of steam coming from under
the front of the fan, both time after a cold start-up and idle of approx 15
min to an hour. It appeared (I was unable to see this happen myself, and
wife's description was less than helpful) that it may have "boiled over" and
ejected the coolant through the reservoir, running down over the exhaust
pipe.

What could cause this? One suggestion was a sticking thermostat, any other
ideas or confirmations? I really can't afford a trip to the garage for trial
and error, and I can change a thermostat myself.

Thank you all...