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Re: engine overheating
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.
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