THis may be too simple of a solution
I have rough idle when i start my 84 300d for about a minute or two and it is a hot summer. I read in the forum that the relay does not shut off with the light in the car and may send voltage to the glow plugs for longer than the three seconds that the glow light is on. Now I give my car a good 10 count and she starts right up with alot less fuss.
hope this works for you too.