1994 Plymouth Voyager, mileage is 150, 00 km.

My van is stalling when the RPMS drop down - when I slow down for a traffic light or slow to make a left hand turn. It doesn't do it all the time but is becoming more frequent. I have had a mechanic run a scope test on it and everything seems fine. They can't find anything wrong with it. However, it is happening so frequently that it is a real safety issue. I saw a similar problem listed and someone suggested that the temperature sensor for the computer may be malfunctioning and causing the computer to think it is too cold to run. Could this be a possibility or is there some other reason for this to be happening. Other than that it is running fine. And every time I have taken it in to a garage for diagnosis, it won't do it. Help!