Saturn Aura 07 Overheating during low speeds

Hi all - my SaturnAura07 3.5 motor has been overheating during low speeds (0-40mph). When I go on the highway, I my temperature gauge will hover around the midpoint and be fine, but whenever I drive slowly (stop/go traffic, cruising in neighborhoods, etc.), my temperature gauge starts flaring high. It gets to the point where when I turn my car off after parking, the car fan runs for another few minutes to cool off the engine.

Any advice for what I should do? I haven't seen coolant leaks (also, I don't overheat in high speeds). My friends think I should take it to the car dealership but I want to check with my fellow redditors before I shell out $500 for a check-up.