Jeep Thermostat Question

I have a 2000 Jeep Wrangler whose coolant tank started leaking in 2006. It was patched and I had no problems until 2007 when a faulty thermostat caused my radiator to blow. The thermostat, radiator and hoses were all replaced. In 2008, I noticed a burnt coolant smell and had the system flushed. This past weekend, I drove for about 20 minutes with the A/C on full. When I turned off the car, steam started coming from under the hood and coolant started pouring from underneath the car. The next day, I drained the coolant and ran it with water for a long time, and it would not overheat. There were no loose or cracked hoses, but there was thick, rust-colored buildup underneath the radiator cap. I then refilled it with coolant and drove for about 20 minutes, and when I turned it off could hear the coolant bubbling for about 5 minutes, but nothing overflowed. Any ideas?