this is very discouraging news...I live in Indianapolis and my car does the same thing. When i'm on the highway the temp gauge drops all the way back down to C the whole time. The most it ever really warms up is maybe 1/4 of the way. I was planning on changing out the thermostat because I'm assuming it's stuck open but since you changed yours already and still having troubles that's kind of a disappointment. I'm not sure what else it could be...sorry.