a lot of times people selling a zx will turn the idle up when selling it to get a good start up to Mask problems. Seen it several times, and youll see people post that you have vacuum leaks, and what 30 yr old car unrestored doesnt. Vapor lock will also Make the car start hard but usually its a bad connection or Bad part. the hard start when the car is warm is a temp related problem. The ecu isnt getting correct temp info. by now your car needs needs attention at the temp sensors, thermotime switch, Aux air regulator, and the electrical connection needs to be cleaned by now. Start by cleaning the connections on the thermostat housing. If its a 5 speed you should idle 700 rpm When warm. the sticker on the hood will tell you..I think, its a gold sticker that has timing info.