2003 Honda Civic Civic overheating, already replaced radiat

On a recent trip, I let the car run too hot thinking the gauge was faulty because I wasn't getting hot air. I learned my lesson. The car started giving white smoke, I pulled over, and saw that the radiator cap was melted and the system no longer could be pressurized. I had the car towed and eventually got it to a relative's trusted mechanic.



He replaced the radiator and also noted that the car was very low on oil. Wasn't sure which caused which or if they were connected, but both problems were resolved. (I use mobil 1 synthetic and changed my oil about a month and a half ago, so I don't know why it was low. Second lesson learned, check fluids regularly)



The mechanic said the problem was fixed. I test drove it on a number of short trips around town, no problem. I did a 20 mile highway test, no problem. I had to get the car home, so I braved a 300 mile highway trip home. Absolutely no problem. Temperature gauge below halfway point, fluids good.



An hour after getting home, I go out for milk. 5 mile drive on the highway fine, but when I get down to city speeds it starts to heat up. Over a mile it gets to red, so I pull over. Even idling and stopped the gauge slowly goes back down to normal. Another mile, its back above red. I give the car an hour and a half and try again, same thing. Good oil check, correct level (well a bit above max) in the coolant backup tank, and later found good level (I think) in the radiator.



So, I get that it would have better cooling at highway speeds and maybe thats why I made the 300 mile trip home. But before that trip it ran around the city fine in stop and go. Now it can't even go a mile?



Head gasket? Water pump? Thermostat?



I thankfully don't see any other head gasket signs. No bubbles on oil dip stick, no signs of oil in radiator fluid, no white smoke from exhaust. The fans come on as needed and appear to be working fine. Any help would be greatly appreciated!