2000 DTS Overheating
 

I have a 2000 DTS that I am at my wits end with.The car started to overheat while in stop n go traffic about a 2 weeks ago. Since then I have had the cooling system flushed, replaced the thermostat,water pump and had the radiator replaced.The radiator was replaced by a shop that only specializes in Cadillacs. They tested the cooling system twice for carbon monoxide to see if the head gasket was bad and the car passed both times.Today I took on the highway and it was fine the entire time that I was on the highway. After I was off the highway for a few minutes the car started to heat up again.I also disconnected the purge line from the surge tank after I got the cooling system was flushed and I had a slow steady stream of coolant coming out.Does anyone have any suggestions ? Should I let the car run for a few minutes when checking the purge line to make sure that coolant is constantly flowing >