94 heating problem's

Hi everyone ! I got a problem with my heater. When it's cold outside (-20C) or when I overpass 50 miles/hour, my temperature gauge go down just over the cold line (normally supposed to be half way between cold and hot temp) and the temperature comes warm (not cold). I have to slow down or stop on the side to wait temperature comes hot. I tought it was a thermostat, that I just changed yesterday. The same problem appears again today. Can someone help me to fix that ? Thanks !

check a service manual and you might have to het a higher rating thermostat to at least heat ur engine up in cold weather. not too hot. when is the last time you changed your radiator fluid , how is your water pump operating? Check for leaks.This might be a dumb question do you put the thermostat in the right way?good luck hope u get the problem fixed.