1997 Sunfire heating problem

I've never came across anything like this before. It's getting colder out and when driving down the freeway my temp goes down to about 120 and blows cold air through the vents when heater is on. This is a 2.2.

Now when in town it heats up just fine and blows great heat. I'm just puzzled why it's cooling off so much I've never had this problem before and we've had this car since 1997.

Any help would be appreciated.

Well a few thing to check and one is there enough coolant in the system. If it is low or has a air lock in it that can cause it to not allow the fluid to pass thru the heater. Or your thermostat can be stuck in the open position while in the city there is not enough air flow to pull the heat off the coolant but on the highway it will pull all the heat off the rad and will drop the temp dramaically. The last thing it can be is a blocked heater core but that is not a very common one unless you have put in stop leak gunk in the past. Hope this helped. I have had my rad freeze up on the highway before and cause a overheat, But unless you live in an extreme northerly climate near or above the CDN and US border then that is prob not the case. Never had this problem with my 96 Sunfire GT yet but have had this happen in my 96 Blazer I used to own.