502mpi's should run consistently at 160-175 on the gauge once they have reached operating temp

raw water cooled or close cooling ?

scan tool reading 155 to 172 can be the normal range for raw water cooled.

Water circ pumps also can cause swings in coolant temps, Basically meaning if its 12 years old or older its time to change it. Also air bubbles in the cooling water flow causes temp swings and off readings