99 neon overheating

My 99 Ply Neon overheats under interstate driving conditions, but not under around town or sitting and idling. I have changed the thermostat and radiator cap, to no avail. Took it to a garage, where he worked on the electric fan controls (at no charge). He also said he could find no indication of a blown head gasket. It doesn't seem to be losing coolant. When it overheats, I have to pull over and keep it idling to cool it down. If I turn off the engine, the radiator cap lifts. When it overheats, I see that the fans are running. I am currently flushing the coolant system. Next is to have the radiator rodded or to just replace it completely. Bought it 2 months ago. Old title was from Michigan, so there is quite a bit of rust underneath. Any suggestions what direction I should go or common problems seen? Thank you.

If fans are running and she does not overheat at idle I would suspect the water pump, But first I would flush the cooling system and make sure the thermostat is good.

Thermostat had all ready been replaced. Flushed the radiator. All the fluid came out looking clean. Car is sputtering pretty bad and dieing now. It's going to the shop this evening. I will report what they find afterwards. Thanks for the input Bear.

the same issue with my sisters dodge neon...
always overheating,, shes taken it to a shop many times but still overheats.
so she handed the broken car down to me:sly:

My mechanic has had it for a week now. I talked to him today and he's been sick, so hasn't started on it yet, but I will report what he finds.

Reasons for overheating: bad fan relay, low coolant, defective radiator cap, clogged radiator, bad Coolant Temperature Sensor (CTS), bad thermostat, radiator fans not working properly, headgasket leaking, radiator fins are damaged, and faulty water pump.

fans are only needed around town ,on the highway your doing 55 and the air is going thru the radiator. sounds like coolant is not moving thru rad at high speeds (water pump) check for kink in rad hose just some ideas...and are you sure its overheating or just an indication problem ? keep us posted

Ok,
I got the car back last week. My mechanic called me and told me to come over, because I wouldn't believe what he had found. He had checked pressure and coolant flow. Everything kept checking out okay. The sputtering and dieing was caused by a wire that melted due to engine heat. He had repaired that. He pulled the fans and shroud to get a better look at the radiator. The fins on the inside of the radiator were so corroded that they were plugged up. Therefore, very little airflow was actually crossing the radiator. You could run your finger across the fins and they would just chip off. A large number of fins were missing, which I'm assuming is the reason it didn't overheat while idling. This was a Michigan car, so I'm guessing the winter salt caused it, but who knows for sure. Anyways, a new radiator, thermostat, fluid, repaired wire and labor for $380. I'm happy, especially since my daughter now can stop asking me when she's getting her car back. Thanks to everyone that has responded.