Engine Cooling/Temperature Gauge

I am having a potential cooling problem with my car (I say potential because I think the temperature gauge may be bad). After the vehicle warms up, the temperature gauge with spike to the very high side of normal. It will then drops to the low side of normal and level out for a while until it repeats the process. When driving on the highway I do not have this issue.

I have replaced the upper and lower radiator hoses, the radiator, engine coolant temperature sensor, ambient air sensor and relay, constant control relay, water pump, and the thermostat three times (The current one is genuine Motorcraft). I have also back flushed the system. In fact, I just replaced the water pump and the coolant that I drained had been in the car for at least a year and was still green.

I do not have any noticeable change in the way the car drives when the temperature gauge spikes and it mostly happens when idling. I am not showing any leaks and the coolant level is where it should be. I am not blowing white smoke from the exhaust, and the oil is not milky. I have had this issue for about eight years and cannot seem to solve it.

Any ideas on what the issue is? I am wondering if my temperature sender is bad or if it could be clogged heater core. The heat has never been that great in car in the nine years I have had it.