Over cooling!

Hey guys, I tried a search through the forum and couldn't find anything concrete so I'll start a new thread. Lemme detail out what I have/know:

'66 Barracuda, stock everything, /6 (of course).

I did radiator work during the summer b/c it was having issues overheating. I replaced the thermostat to the stock 160 and after that in never overheated. The radiator held pressure with a water test, and it was flushed by a professional so I'm confident that's fine. The thermostat is brand new, only 2000 miles on it tops(I regularly drive across the state). All this work was done in August.

Recently, especially after this past weekend(outside temps low 20's high teens), the thermostat temperatures drop drastically, and stay cold. Normally it stays just on the inside of the operating range on the cold side, but this time it dropped below. The defroster reflects the temperatures on the gauge(when it's very low the heat coming out is lukewarm at best) I'm not sure exactly what's causing the issue. It starts out pretty warm as well, it's only after I'm on the highway for 30+ minutes. It doesn't do this when the outside temps are >40-ish degrees.

I was thinking of putting in the 180 thermostat, but not positive. If I put any extra fluid in the radiator it gets immediately dumped through the overflow.

EDIT: felt it was important to note, I was getting ~17mpg this past trip.