engine overheating

my 1995 ford windstar runs great, then begins to overheat. I replaced the thermostat with another that I found out was bad, then replaced it again. It ran fine and now is overheating again. At times the gauge fluctuates cool then hot then cool then hot. Are the better after market thermostats that bad that they continually fail or is there another cause for the overheating?

For 1995 3.8L, it looks like 195 degree F is the OE temperature.
Rockauto sells Stant and Gates, and has a Motorcraft Thermostat listed.
You might try your local FORD dealership.....not an expensive part.
I don't know why you would have multiple bad thermostats.

Are you having to add coolant?
If you are having to add coolant......it is possible that the same leak that is letting coolant out on the warm cycle is letting air come into the motor coolant loop during the cool cycle.....and even a small air pocket can cause the temperature to cycle high.

Also, after replacing the thermostat......you might have to "burp" air pockets out of the system.......if the temperature shoots up.....shut off the motor and let it cool.......add coolant as needed........and then run it again.
Once the air is burped out, it should not shoot up.

Verify that your radiator fans are working........it is not unusual for the low speed dropping resistor to break......causing you to have only high speed for the radiator fans.
This will cause the temperature to go higher before the fans come on.
There is a temperature that the radiator fans come on at low speed.....through the dropping resistor.
Then, if the temperature climes to a higher setting, the radiator fans come on at high speed, bypassing the dropping resistor.

If you turn on the A/C while the vehicle is not moving.....the radiator fans should come on at low speed, and stay on.
If they do not, then there is a problem.

The temperature should not go up to a high level while cruising down the highway......the air that is naturally flowing through the radiator should keep the engine coolant temperature pretty constant.
It is when you are not moving that the temperature will vary.
What I do in stop and go traffic is to turn the A/C on......you can do this with any setting that includes defrost.......as any defrost setting will turn the A/C on.
This keeps the radiator fans running when you are moving below some minimum speed.....keeping the motor at a cooler and more constant temperature.

Were the bad thermostats stuck open, stuck closed, or opening at a wrong temperature?

Thermostats work on the expansion property of a wax when it warms to it's melting temperature. The expanding wax is contained in a cylinder with a piston, so the piston is forced out to open the valve, allowing coolant to flow. When it cools, the spring pushes the piston back into the cylinder to close the thermostat.

I think the main failure mode for a thermostat is when the wax leaks out (stuck closed). When operating an engine with a thermostat that is leaking wax, you should see an gradual increase in the operating temperature over time before the engine overheats.

A thermostat that is stuck open would cause the engine to run cold, so I doubt this is your problem. Corrosion or a bad spring would be the main culprits in this case.

I'd agree with Wiswind, that the OEM is probably a better thermostat, and not much more expensive.

The rapid change in temperature is not likely due to the thermostat. Instead, as Wiswind has mentioned, it's probably from air in the cooling system. In addition to his suggestions, consider that overheating is often a symptom of head gasket failure, and the '95 Windstar is known for head gasket issues.

Replacement thermostats are just LOUSEY (spelling??) ... no matter where you get them. And they fail in a variety of ways ... usually they just get lazy and are open too much too early ... but I have had them just fall apart and plug the coolant flow.

I usually change the screws (that hold the thermostat housing in place) to studs and nuts just to make changeout easier and quicker.

This would be rare, but also possible due to the age of this vehicle. Cool then Hot back and forth ... back and forth.... I'd be willing to bet its cool when you are driving it and hot at stops. How sharply does the temp gauge rise when you stop? If it rises dramatically fast on the stop, your catalytic converter(s) may be bad and/or glowing red. Though if you were dumping too much fuel (what causes them to go bad) you'd usually have a computer code (CEL light on) for this, though it doesn't always work how its supposed to and you can have a problem without a code easily.

When my cat was bad the temp gauge rose to overheating within a matter of 1-2 seconds tops... every time I stopped. Verify what everyone else said first though before thinking about this.