When I run my AC when I'm in stop-and-go traffic.

When I run my AC when I'm in stop-and-go traffic, my temperature gauge will skyrocket to the "red zone." My car will start to smell like burning rubber/plastic, and if I don't catch it in time, orange liquid will come bubbling over and spilling onto the ground. If I'm driving on the highway though, it's completely fine. I know the car is essentially overheating. Any ideas on why and how to fix it?