Which thermostat?

Two months ago I've replaced my old 180 degree thermostat with a new 195 degree one, but since yesterday it seems already broken. The temp of the engine is much lower since yesterday and the heater doesn't become so warm anymore.
So, what could have caused this?
My guess is that I need a new thermostat. Should I buy a 195 degree one again? Or a 180 one?
I have a '91 Caprice 305.
Thanks.

When is the last time you have flushed your heater core?

Tom

The 305 came from the factory with a 195 degree thermostat.

Strange situation. I've replaced the thermostat with a new 195 degree one and the heater is now much warmer and the temperature meter reads a normal temperature now. So, I thought the old thermostat was broken. But I've tested it in boiling water and it opens and closes perfect. So, what's going on here?

A 195F degree thermostat that is open at 212F is not a positive test that it is working. It might open too early. Did you measure the temperature as the water approached boiling and before the thermostat opened? It would seem however that the problem is now solved.

I'd still flush the heater core if that's never been done before. After 27 years there are likely a few stroopwafels stuck in there.

when i've tested thermostats, i put my wifes cooking thermometer in the pot to verify the opening temperature. Don't tell her....

Your secret is safe with me. I've cleaned auto parts in the dishwasher.