Honda Civic 96 overheating issue?

My car has been overheating for a week now, but it only overheats while I am at a stopped sign, and then it goes right back down. Never reaches the red line. When I turn on the heater, the temp gauge also goes back down. I have changed the thermostat, radiator, and water pump, plus timing belt, but it still overheats. Fan that is connected to the radiator (passenger side) is spinning. Now the overheat doesn't happen everytime I stop. Today, it has only happen twice. Gauge goes up just a little, goes right back down when I start to drive. Can it be the head gasket or anything else that I am missing? No visible leaks as well, and I did the pressure test to checks for leaks.

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