Totally having the same issue here, so any light on this would be fantastic.

Smoke is blue/white and reeks of unburnt diesel. Car just crossed 369,768 miles (1982 300SD). Started to stumble and run rough at idle; what used to be a barely glow car to start has turned into something that does not want to start in 50 deg. weather.

Symptoms are the same, huge cloud of smoke at idle, and part throttle. Get on the highway, all smoke stops. Go back to idle, and whammo, fog machine.

Mileage has also suffered; I think I managed to barely get 20mpg at the last fill up. No access to a compression tester, but the car's power is just like it always is on the highway, and she takes off quite smartly.

I'd be very, very curious to see if this boils down to an injector issue, valve stem seals, or what have you.

Please keep us posted!

-James