If all else fails it could be a plugged catalytic converter. I have a 1987 300TD that had this problem. If you cannot get the rpm's up when parked, try removing the exhaust crossover pipe. If the rpm's suddenly jump up, the exhaust system is plugged.

A few years ago I also had a fuel thermostat that sucked air. I bypassed it and it runs fine in cold weather.

Good luck.