You could have a coating inside the cooling system insulating the engine from the coolant. Maybe try a good chemical flush compound, the complete flush, and perhaps a water wetter after that.

Boy Eric if you have replaced the hoses already I don't think it is them. If you can sit in traffic and it dosen't get hot then its not the fan clutch, at speed the air going through the radiator is doing all the cooling not the fan. If you are not having to add coolent then you don't have a leak.

So where are we. Flushing the cooling system, I'ed take it to a shop that has a power flushing system. They do a much better job at cleaning and they can determine if there is any cloging in the radiator or engine. One last thing after the flush have them do a pressure check of the system. Remember you can have an internal leak at a head gasket or small crack. If everything checks out. Take it out and run it, if it still gets hot, replace the water pump and add 40 below coolant additive you can find it in Ecklers and many other catalogs, also replace the thermostat with a 160 or 140 degree unit.

Normally the problem with overheating is from sitting in traffic or ideling a long time. I remember being in a parade and having to run the heater in the middle of the summer to get enough cooling to keep it from over heating. At speed the air flow through the radiator and the water pump running faster should supply plenty of cooling.

One last thing make sure there is a thermostat in the car. Some people think if their car is overheating, by removing the thermostat it will run cooler. Not true the thermostat is also a flow regulator. If the coolent circulates too fast it does not have enough time to cool in the radiator.

In a former life I built dirt track cars. We diden't run thermostats we used plates with a one inch hole in it to regulate coolent flow.

Good Luck Let me know how you make out.

Thanks Glenn