had the exact same situation with my 83 300SD. I first tried replacing all the plugs, to no avail. Then, i took apart the relay and found that the relay connection points had carboned up. I cleaned them up with some sand paper and all is good. Oh yea, whenever you touch anything on the glow plug system you will think you have solved it until you pick up a friend or girlfriend etc, then, and ONLY then, will you realize you havn't solved the problem. And then you crank forever and make your car look bad. Must have done that 10 times, everytime thinking "YES! i fixed it, only to figure out i hadn't
) been a few months and several trips with my g/f and it hasn't re-appeared
