Vintage Cars: temp issues, correct temp, expansion tank

Question
I have an '85 500 SEL, its been running a bit hot when sitting in traffic with the air on (110c and even a little above) I did a very complete flush job on it which didn't help.  it had the radiator replaced last year so I know that's not the problem.  I decided to get a thermostat rated for a lower temp.  I found one rated for 75c and installed that (made by Stant).  Now, the engine warms up and goes almost all the way to 120c then VERY quickly drops to 80c and stays right there.  I drove it for an hour today and like I said, initially it climbed all the way to 120 then bang, dropped like a rock to 80 and stayed there for the remainder of the trip and never went back up.  initially I thought maybe there was an air bubble but I've seen it do it every time I run it now.  I also thought maybe I put the new thermostat in upside down but I check it and it's in right.
is the new one just defective maybe or could there be something else wrong?
thanks so much for you time and effort!

Answer
the correct thermostat will have a small bleed hole and a small arrow that points "up"  Correct temp is 80 degrees
T-stat in backwards or a cooling system that needs "burping" can cause overheating. So can a bad expansion tank cap,But  the sudden temp lowering  would suggest T stat issue .  Certainly is possible to get a bad one. There is also a small communication passage between housing and block ,is that clear?  It sometimes gets clogged