Basic troubleshooting: Blocked radiator?

This will cause overheating at highway speeds. This is because if the radiator is blocked the engine, which at highway speeds needs more coolant flow, is not getting the flow. So it overheats.

The classic sign of a blocked radiator is the lower radiator hose collapsing. This is difficult to asses on these cars because, at least from the factory, the bottom hose has a strong wire frame to keep it from collapsing.

A blocked radiator usually is blocked only in the bottom. Yes, the cores will be clogged a bit, but the real problem is the crud collecting in the bottom of the radiator. It is already full of your transmission cooler so there is not much room there for crud to collect before it starts restricting your coolant flow.

But in this case you may be fighting several problems. I would start with having the radiator boiled out AND replacing the water pump. After that it time to start checking the other water pipes and perhaps the thermostat for a build up of sluge that is restricting your flow.