Bad thermostat

93 Caravan, 3.0L
The last 3 thermostat work well for 4 weeks then they start to cool too much. When I install a new one, it will keep the temperature right on target for four weeks. Then slowly I notice that it takes more time to warm up and that the temperature fluctuates up and down. When I remove them, they are slightly opened at room temperature.
I installed the recommended temperature. I think it was 192F.

Is there something in the coolant that can affect the thermostat?
Why would they fail so quick?

Thanks

93 Caravan, 3.0L
The last 3 thermostat work well for 4 weeks then they start to cool too much. When I install a new one, it will keep the temperature right on target for four weeks. Then slowly I notice that it takes more time to warm up and that the temperature fluctuates up and down. When I remove them, they are slightly opened at room temperature.
I installed the recommended temperature. I think it was 192F.

Is there something in the coolant that can affect the thermostat?
Why would they fail so quick?

Thanks
They reccomend that the anti-freeze be replaced every 2 years so there must be something to the nature of it not doing what it supposed to do after a period of time/miles. The other factor is where you live, where I live in the North, because of the harsh temperature changes my anti freeze gets a hammering. If you are changing it yourself, remember to dispose of it in the correct manner. All the best Sam