At idle or slow traffic my temperature gauge rises and my heater blows cool air

Hi, I hope you know what causes this. When I am at idle or slow traffic my temperature gauge rises and my heat turns cool. So I put it in neutral and rev up the engine to around 3800 rpms for a little bit and the gauge goes back down and the heat comes back on. Everytime! I already changed the thermostat to an AC-Delco so I know its not that. I have checked the belt and the coolant level and they are fine. I thought I would change the radiator and while I was in there the water pump also. But I would hate to spend the time and money and find out it is something else. Is there anything else I can try or have you seen this problem before? Thank you. Sincerely Marty.