Temperature goes up and down

My car is a 1996 Mazda Protege 1.5L, automatic, and it's about 132,000 for the mileage. Recently it has been replaced with a new distributor + cap, fuel filter, sparking wire and timing belt. I also took it to have oil change and transmission fuel change. But then now when I hit the brake and stop the car for a longer time (maybe 1-2 mins waiting for traffic), the cursor in the H/L gauge on the odometer would point up into the high temp region. While I keep running after I stopped, the cursor would point back down to the normal temp region. I never had this problem before the car had oil change and transmission fuel change. I called the auto repair shop and they said that it's nothing related to those parts. Therefore, I don't know what happens to it and I wanna know if it's a serious problem to the car. Thank you very much.

P.S. I am running the car with the air-conditioning on.