454 over-heating at idle.

I have a 1979 GMC c3500 2wd 1ton dually. It has a 454 with an Edelbrock 750cfm 4 barrel carb, Holly aluminum intake, Long tube headers and a stock distributor. My timing is set to 11 degrees. This just started randomly about 3 weeks ago, at an idle my truck will very slowly start to over-heat; meaning if I let it idle for 5-10min it will climb to over 220 degrees. While driving it runs fairly normal (175 degrees like it always has before). I have so far replaced the factory radiator with a new one from AutoZone, I have replaced the water pump, radiator cap (16psi), radiator fan (flex-fan), new fan shroud, I put in a Mr. Gaskets 180 degree Thermostat and I’m using an overflow with the new radiator. I’m not losing any fluids. I have a hard-line temp gauge and I have a temp gauge in my radiator cap so it is not my gauges. I have no anti-freeze in my motor oil and I have no motor oil in my anti-freeze. For the most part the truck runs good other than some times while accelerating the motor sounds like its popping and there is a loss of power. (I had an Edelbrock foam air filter and one time while going down the road it made some popping sounds while accelerating and I noticed that the foam air filter melted a little, so I changed the air filter). Normal day to day driving my trucks runs good. I need to know why my truck is over-heating at an idle. What checks can I do? I don’t know the mileage on the motor but seems to run healthy at an idle, and driving down the road for the most part.