1998 Pontiac Grand Am cooling

I first start the car up it and drive it until it reaches operating temperature it then goes approximately 20 degrees above normal and then goes back to normal and it doesn't happen again until I turn the car off, let it cool down, and drive it again. It always stops at the same spot and always goes back down to normal. Why is this?
It has a new radiator, thermostat, coolant, no head gasket leaks. Mechanic has insisted that the water pump is working fine.