If stopped in traffic for over a minute, you need to put the tranny in neutral. When stopped the torque converter is slipping creating heat and at idle you are not getting enough air flow.
Since this is a new problem and it does not do it at speed with the trailer, you need to clean your radiator/CAC better. Not sure if you have a crank case vent tube, but if you do you may need a slobber hose added to it to keep oil from getting into the radiator's fins. Do a search on cleaning and slobber tube for more info.