I have now replaced the distributor cap and rotor arm, and the coolant temperature sensor. The warm up cycle is much improved, and the car generally runs much better, apart from what I now beleive to be one intermittent issue, which is that the fuel mixture is being intermittently enriched. Sometimes at standstill, the car idles beautifully, and then the RPM increases to 1500, accompanied by black smoke coming out of the exhaust. This intermittent enrichment also happens when the car is moving, indicated by hesitation and the occasional stall, and again black smoke coming from the exhaust. I have checked for vacumn leaks and could not find any. I am going to have the fuel pressure checked, and also the air mass potentiometer, by a technician. Do you think I am thinking on the right lines to resolve this issue, and does anyone have any suggestions for any other items to check. Thanks in advance for any guidance.