The rad can look 'fine' all day long but it is impossible to confirm this visually. The rad 'fins' can be fine i.e. what you can see looking from the front through the rad however, it is the tubes inside that crud up which you cannot see without the rad being dismantled.
If you consider this, driving around town the engine is not working and so the flow of water through the rad is minimal, please keep in mind that these engines will tickover all day long without the standard fan such is the heat loss through the rad.
Once you start working the car the heat needs to dissipate quicker hence the need to flow more water/air. Invariably higher speeds down the motorway increase the engines workload but any restriction will prevent coolant being exposed to the airflow.
This is worse if climbing a hill as the car tends to be slower but the workload may have tripled.
Your post claiming all is well may have been due to the fresh coolant that you put in disturbing sediment in the rad or moving an air lock which if it has returned would indicate perhaps that the system is being pressurised by a weakness in the head gasket indicating failure in short order.
I would have started with the cheaper things in this order:
Start with is the gauge accurate and is the engine actually overheating? Looking for leaks, fan belt, rad fins (airflow obstructed?), air locks (all hoses hot including heater hoses?), thermostat, rad cap (not maintaining pressure allowing coolant to boil off), water pump, collapsing hoses (under load bottom hose can be sucked flat), reverse flushing rad (rarely a 100% fix), viscous coupling (remember this sometimes is needed when engine working hard even if there is some airflow from the forward motion of the car not just at idle), recore/replace radiator (impossible to check without dismantling but worth doing anyway), head gasket (chemical test coolant), and unlikely but possible incorrect timing.
Someone else will hopefully throw in anything I have not thought of.
Hope that helps
regards
Dave
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