2000 Other Mitsubishi Models Boiling resovior

I have a 2000 Mitsubishi Montero Sport and I just as I pulled up into the driveway this afternoon, there was this boiling sound under the hood. I lifted it and found the coolant overflow resovior boiling almost to the top. It now went back down to the max fill line. My truck showed no signs of overheating, even though I live in Arizona and it's 111 today and I've been running errands for three hours. I just paid a mechanic to drain and replace fluid in my radiator and do a thorough tune-up. What do you think the problem may be? Oh, the coolant is dark also and there is a burning smell.