I just bought a 1994 Mustang GT. And the Stock Fan does not appear to be spinning when Im idling. I have checked a couple times under the hood, and it never is spinning. No matter if the car is at 180 degrees or 230 degrees.
To test the function of your fan, apply 12v to pin 14 of the CCRM for low speed, and ground pin 17 for high speed. It could also be a bad fan, since there was a recall on them, which added a circuit breaker to keep the wiring from frying if the fan seized.
To add just a little bit more info, see this tech-note. Start at the portion labeled If a car runs hot at slow speeds or while stationary, but cools well at speed (40 mph +):
There are also links to CCRM diagnostics and wiring up manual fan switches in that tech-note.
Usually if you're driving and the temp drops, it's more of an indicator of the fan not working. When you're stopped the fan draws air across the radiator, but when you're moving, that's what forces air through. Another quick test if you have working AC is to turn on the air to max. It's supposed to turn on the high speed fan.