1993 mustang LX heating problem

the temp indicator in my dash indicates that the motor doesn't reach the normal operating temperature. This is happening when the car is in movement. On the other hand, if the car doesn't move, the temperature will be reached, but it will drop once I move fast. In addition, when the indicator says the motor isn't hot enough, there's no heat at all that comes from the ventilation. Heat will be back when I let the car run without moving and let the temp reach normal status. Have replaced thermostat,pressure cap,antifreeze,have noticed proper coolant flow.Still takes long time to get to temp from cold,then cools off when under movement

What is the temperature rating of the thermostat? When was the last time the cooling system was flushed?

OK here's the story,the car has a 2.3L ohc eng,but the thermostat that is supposed to fit(according to Autozone),is too small.,what was in it was for a v8 eng.I replaced the thermostat with an identical one rated about 190f.From cold,if I let it sit idling for an hour,the engine temp reaches sufficient level to make the heater blast you out.After about 30 min of driving the engine temp drops and so does the heater temp.Seems if there is no air flow over the radiator(like sitting still) the temp is fine,but start moving and the engine cools down too much.Was thinking maybe the fan runs too often(?)

There is air flow through the radiator when the car is moving, therefore the fan doesn't matter when the car is moving. The problem is that the motor is overcooling. The thermostat is either opening too soon or sticking open. Put the thermostat in a pot of water on the stove and heat. Use a candy thermometer and measure the temperature of the water when the thermostat opens.
Keep in mind, new parts aren't always good parts.

As long as car is sitting still,the engine remains at good average temp,whenit begins moving is when the temp drops,come to a stop again (like traffic)and temp rises back up.New thermostat made no difference,checked old thermostat in boiling water,and it opens at 190o..Heater output is high as long as the car doesnt move.

How cold are the winters where you are driving? Try doing what the truck drivers do, block 1/3 to 1/2 of the radiator to limit air flow, restricting some of the radiator's cooling capacity. Whatever you use to block the air flow must be secure so it does get stuck in any moving parts. I would ideally use a piece of sheet metel, punch holes in the corners and attach to the radiator with wire ties. Others have luck with using cardboard, canvas, or plastic. Make sure the coolant level is topped off. Low coolant level can also cause this.

As long as car is sitting still,the engine remains at good average temp,whenit begins moving is when the temp drops,come to a stop again (like traffic)and temp rises back up.New thermostat made no difference,checked old thermostat in boiling water,and it opens at 190o..Heater output is high as long as the car doesnt move.

I have a similiar problem with my 93 l/x 2.3 .It seems that the 4 cylinder shares the same radiator with the 5.0 .I'm upstate new york and it's barely as cold as it can get right now,and the gauge seems to be reading cold.First of all i dont like the gauges.They have to be looked at an angle to see the proper reading.Second,they don't show the actual temp between 130 and 270 degrees.