1997 cadillac deville overheating

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About 2 months ago, as I got off the Interstate and started up a country road, I saw that my car was overheating and the A/C shut down. The readiator was busted. I have since replace the radiator, thermostat, water pump, radiator cap, cooling sensor, and had my radiator flushed. The car will overheat in stop-and-go driving situations. At night when it is cool, it is very hard for the temp to rise on the car. First, I had to put coolant in the car about one a week, now it is about every 4 to 5 days. There is no water in the oil. I took it several shops and they said they couldn't find a leak. Where is this coolant going, and does anyone can figure out this problem.