I have a Mercedes 400SEL 1993 model which drives very well. The problem which i has is the following: it starts immediatly when the engine is cold, but if i turm the engine off and try to start it again, it just cranks and does not start. I have to wait untill the engine cools off to about 40 degrees celsius, and then the engine starts right away. I have replaced the spark plugs, crank shaft sensor, fuel pump relais, and both ignition coils. After having replaced the ignition coils, the car started pwrfectly for about 1 month, untill the problem came back. What could be the problem. Thanks