I have a similar problem. My 99 E300D 160K does not like to run smooth when cold. Before the cold set in, it started and ran beautifully. Now it will only do that if I use the block heater. Yesterday, (cold out) I unplugged the heater for 4hrs. and tried to start the car. It started but would only rev up slightly above idle, ran very rough and smoked up the entire neighborhood, (well, maybe not that bad). I kept it running this way until the engine temp went up and then it smoothed out and ran normal again. I disconnected the glow plug wiring harness and put a volt meter on each of the contacts. The circuit closed each time to 12.5 volts +/- and stayed on for about 30 seconds after the glow plug light went out. This was with the engine warm from the block heater. Any suggestions on where to start with diagnosing this problem? And what it might be?

Thanks,
Mike-in-Sky