2000 Jeep Cherokee 4x4 Sport - rusty coolent

I have a 200 Jeep Cherokee Sport 4x4. It has about 80,000 miles on it. All of a sudden it overheated and the temp guage was wacko. So we replaced the thermostat. In doing so the radiator was flushed only to find a rusty-sludgy colored liquid in it. All was fine, then after about 2 weeks I went to have an oil change and when they opened the radiator again the same rusty-sludgy stuff in it. I had it flushed again and then afterwards all of a sudden the temp guage is all the way up again. I am puzzled???????