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. When I lift the hood the coolant is boiling into the overflow tank. 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 water pump, new head gasket, cooling system flush again. All of this was done by a mechanic starting with the easiest and cheapest and working up to the head gasket. What is causing this problem?
I have posted this in multiple forums and most replies involve problems directly with the cooling system, however, one reply said it could be a plugged catalytic converter. The thing is, it just passed an emission test. Could this be the problem?

 

Anything that makes the engine run harder would make it run hotter. Make sure it's idling right and not trying to speed up constantly. It also may have a lean mixture causing it to run hot. Check your spark plugs and see if they're kinda white cuz that's the sign of being lean. Fuel injectors, filter, regulator, pump, or o2 sensor could cause that. I'd also check to see if there's any codes.