'99 Dakota V6 Coolant/Heating Problems

I have got a '99 3.9L V6 Dakota that is giving me some problems. When I drive it around the city - slower speeds - the temp gauge will rise to the redline then drop to normal operating temp, may rise again before it truly normalizes out. During this time if I am running the heater/defroster, it blows nothing but cold air. I have changed the thermostat and the coolant temprature sending unit and still have the same problem. Any ideas or where to check next? Thanks in advance for any advice...

I had the exact same problem with my 99 and to my surprise it was the coolant level was extremly low. My thinking is an air pocket would moved into the heater core and that would explain the cold air and without enough coolant circulating that would explain the near meltdown. After adding just one gallon of coolant directly to the radiator and the rest to the overflow bottle the problem went away. I drive in Washington D.C. rush hour traffic daily with no problems since. Hope this helps.