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I had the exact same problem.
When you are overheating, turn the heater full blast and watch if the temperature goes down. If it does, it's the radiator.
It is probably clogged/corroded and cannot keep up with the cooling at freeway speeds, regardless of how cold it is. I had to drive through the summer like this, 90 degs and heater full blast(fun...). I also drove in the winter in ~30 degrees. That huge difference in temperature only gave me an extra 5 minutes before it would start to overheat again.
I finally got a Koyo and there have been no overheating problems since, finally. I did change the thermostat, cap, and water pump but that made no difference. Try getting another radiator to test it out but as always, you can't always trust used parts. Also make sure your overflow tank is clean inside or else it will suck up particles and ruin the party thats going on in your radiator. That is how mine got clogged, making my track day a track morning..
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