1997 Toyota Corolla Temperature Flactuation

Hi, I am experiencing temp. Fluctuation on my car. This happens when I am stuck in a traffic light or on a full stop elsewhere and suddenly, my temp meter reads intermittently high almost over the red color of the temp meter. As I drove back the street speeding more than 60kph, temp reading will be back to normal. One time during a long drive (80-100kph), temp meter abnormal flactuation was also observed. Have seek a mechanic and was advised to replace my clogged radiator since the coolant level in it is normal, no leaks were also found and my radiator fan is operating normal. But still, the same problem was observed. Thermostat was temporarily removed, have also checked the temp meter and it's working normal. What else do I need to check/replace to solve this problem?