What can it be?

Hello I’m new here, and I’m a proud owner of a ford escort 1.8 16V Ghia! is a great car but I have a problem! if the essi is cold and i am driving at a traffic light and i take the walk out so that it is in empty run, the speed goes down abruptly and sometimes goes out! I have no idea what it can be! I hope you can help me! MFG bobenny