My 1997 Plymouth Neon is Seriously Overheating (on occasions)

I have a 1997 Plymouth Neon which is seriously overheating on occasions. Last December, I drove from Washington DC to Milwaukee, with an overnight in Dayton, Ohio. The second morning, I had only gone 100 miles when the engine temperature gauge spiked because I had lost almost all my coolant. I carry coolant and filled it up immediately. When I got to Wisconsin, I replaced the thermostat. Everything was fine for two weeks in the sub-freezing climate of Wisconsin. On the return trip in early January, I checked the coolant before leaving and the morning of the second day of driving, but the same overheating situation occurred later in that second day. Since then, back in DC, most of the time I drive it short trips and everything was fine. Last Saturday, I drove about 100 miles and coolant had started to back up in the coolant reservoir and was dripping from somewhere. I took it to the garage today. They did a pressure check and could find no leak. Does anyone have a suggestion for what could be causing the problem?