I just noticed you said autolight plugs.
odds are good that many of them are bad. autolight plugs do not like mb's for some reason.
they fail as soon as they are installed in the car.
pull them all, (I know, I know, huge pain on this motor) and install genuine bocsh plugs.
you will thank yourself with much better starts, and not have to think about the plugs for a while.
also, when the plugs are out, get in there with a ream tool. carbon builds up on the plugs, and can insulate them so that they do not heat the chamber at all.
oh, is your EGR functioning? how dirty is your intake? check out my thread about my new to me 87TD, I have a nice pic of my new EGR gasket... I recommend you get one like it.