the ac compressor puts a significant extra load on the motor, so motor has to work harder, creating more heat. at highway speeds, there is a much larger volume of cooling air flow through the radiator helping keep it cool. I suggest you give an inspection to the cooling system. I believe the Honda has an electric fan that is thermostatically controlled, do you hear it running when the car is overheating.? If not, check the fan and thermostat. Also, since the car is almost 20 years old, the radiator may need a little service.
It is possibly the overdrive solenoid located within the transmission. It prevents the transmission from downshifting and causes stalling at low speeds. It's a cheap part but a real hassle to change.
Check and see if the fan cluch is good. Your fan. At higher speeds, the fan is not needed due to the high airflow. At low speeds and idle, you need the fan to operate to cool the car.
can't say for certain but it sounds like there could be a lean fuel mixture or a timing issue. If you are lean at freeway speeds, the engine will get hot. Once at idle, the bike may be returning to normal fuel/air mixture & cooling.
make sure there's enough coolant in the radiator.
Replace your fuel pump relay.
It can be freezing up. Have it checked out. Check the Radiator Fan Control Unit. When the a/c acts up on Honda cars, this control unit is usually the culprit as unbelievable as it might seem.
The moving air increases the rate of evaporation of your sweat, so cooling you down.
It sounds like u need to replace the thermostat. At slower speeds the air flow through the radiator is minimal, but at higher speeds the air cools the radiator more. This only happens if the thermostat is not keeping the water in the engine block long enough.
Check your radiator fan. The reason the vehicle cools when driving is because the airflow is much higher, cooling down the motor. Simply turn your t-bird on, let it run for a little while, and check that the radiator fan goes on. If it doesn't, there's your answer.
check your fluids it is probably low on antifreeze and if it still does it change your thermostat that should fix the problem
The car.
If you are saying that at highway speeds it is running at 40 degrees celcius (and it isn't extremely cold out), I would say that your thermostat is sticking open. I know you put a new one in, but every now and again a new thermostat will either stick open or shut. If you do the work yourself, what you can do to test the thermostat is to remove it and put it in a pot of boiling water, you should see it open up. Now remove it from the water and watch it, it should close up in a short amount of time, if it doesn't you need a new thermostat. If it is working, I suggest making sure that the cooling system is completely full with no air in the system. If this doesn't work, you might want an expert to take a look at it. Hayden