2002 Pontiac Grand Prix

My problem with my car is in the instruments panel shows it is too hot but that false. AFter 15 minuts I start the engine the needle temperature rise to 250 degrees. Last time I check the real temperature and this was only 190 in th tool but in the instrument panle was 250. While the car keep running the needle temperature keep under the 200 but when I make a stop for a least 3 minutes this go up to 250 or more.

I changed the temparature switch, thermostat, radiator cap but the problem continues