Daewoo Espero 1996 Problems

IT DOES sound like fuel vapourisation and there could be several causes, but I doubt dirty injectors would be one of them. I would expect dirty injectors to cause poor running at all times and not just when the mercury rises. You could try running on premium unleaded and see if that helps. You could also check the fuel pressure regulator and the fuel pump. And look for places where the fuel in the fuel lines could be subjected to heat that might cause it to vapourise on the way to the engine.