I currently have Bosch Platinum +4s in it, and when I looked at them this evening the electrodes were black and slightly oily. They all looked pretty much the same, with #3 a tad darker.
I'm probably going to get a set of Supers tomorrow and see what that does -- the electrode base was the correct color, so I know the running mixture is OK -- duty cycle floats between 50 and 54% at idle, the roughness isn't connected to any one cylinder by leaking down the injection lines, secondary ignition stuff is fine (almost new) and the plugs are all the same.
Funny thing is that it idles smoothly just off the highway, only starts rumbling when it's driven on country roads at low speeds for a few miles. This makes me suspect the plugs -- I know now that resistor plugs aren't a good match for this car, didn't know that when I put them in.
The other possibility is that I need new valve guide seals. It uses about a quart in 1500 miles, part of if a front cover leak, part a valve cover gasket leak (needs a new gasket), but I think too much may be going down the valve guides.
I will also check the O2 sensor, since I ahve no idea when it was changed last.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!