Escort runs great with heater or a/c on. It runs rough and sometimes dies when they're off. What could be the problem?
Kind of a shot in the dark with the information given, but here is where I would start..
When you have the A/C on, the ECU will typically raise the idle to compensate for the extra load. I am thinking you have a dirty throttle body, that is not allowing enough air to get through to allow it to idle properly. When the ECU raises the idle, it is now letting enough air in to idle properly. I would clean the throttle body with carb cleaner and a toothbrush. There is likely a ring of carbon around where the butterfly valve closes.