Might be a tired glow plug. If the idle is rough long enough, loosen the injector lines one at a time until you find one that doesn't change idle quality. When you isolate the injector, swap with another cylinder (don't forget new heat shields) and see if the problem moves to the other cylinder. If it moves, the injector is bad. If it doesn't move, the cylinder or IP is bad.

Sixto
87 300D