2001 Plymouth Neon LX overheating

Recently purchased 2001 Plymouth Neon LX. Driving on surface streets it does fine, but when I get on the interstate after about 10 minutes the temperature starts to rise and within about 15 minutes it is at the top of the gauge. Pulling over and idling does not cool it off, neither does exiting the interstate and driving on surface streets. The cooling fan is functioning properly. Here is a list of repairs attempted to fix problem: cooling system power flush, new thermostat and radiator cap, new radiator, new head gasket, cooling system flush again. What is causing this problem?

plugged cat

plugged cat

It just passed an emmissions test. Could it have passed if the catalytic converter was plugged?

Also, I forgot to mention that I have replaced the water pump.

Thanks for your response. It is the only one so far that is thinking beyond the cooling system itself, and I have replaced everything I can think of that is directly involved in cooling.

hm.. have you thought of the PCV valve? those things working properly are supposed to keep the temp down also.

sorry for the late reply too, btw.

oh, ok, so...when youre driving on normal streets nothing happens.. and only on the freeway.. how does the oil seem? bits of metal? coolant slowly leaking away?