Coolant temp gauge

Attached is a picture of my dash coolant temp. gauge. While driving my 1996 ford f250 7.5 v8 my gauge reads around the "N" to the "O" but while stopped at a stop light or drive through it rises to the "M" to "L" but when i start driving again it drops back down to the "N" or "O". it is 90 degrees out side with the AC on and i wasn't towing or traveling up hill it was city driving and i also can here a difference in the engine as i step on the gas it feels like it take more pressure to push the gas petal also. any idea or it that normal?
Thanks



