My low milage 07 520d Msport used to judder on cold mornings, as if the engine was about to fall out, the revs would dip up and down, as soon as the car warmed up all was ok. The usual elementary checks were made. filters, injectors, compression,glow plugs,etc and all was ok

The local BMW dealer kept it for two days during hard weather leaving it outside, and said the problem was in the Dme control unit that manages all the fuel and air settings.

This was replaced and it went away for a while. But now its back and this time they are looking at the autobox torque converter.