Overheating on highway, cools back down when driving in city traffic.

Car: Toyota, Corolla, 1987, 1.6L CS - Back to Fix-It - Toyota Repair Manuals

Q.Overheating on highway, cools back down when driving in city traffic.
I presume either the radiator is blocked/corroded, thermostat is broken or waterpump is faulty. I am hoping Head Gasket is not cracked. Can you please advise how to determine where the problem is?

I already checked/done: Replacing the engine coolant. Fans work fine. Not using A/C.

Answer You problem is an unusual one, its usually the other way around.
Only 2 things i can think of that could cause it,
one, the thermostat jammed wide open, this will allow coolant to flow through system to fast (with highway RPM's) to pick up the heat from the engine.
For some reason the fan has been wired to spin backwards. This would stop air flowing through radiator at higher speeds but would flow air through in traffic. Has there been any wiring done on your car recently?

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Overheating on highway, cools back down when driving in city traffic.