mine is exactly the same, and a blip to 1k rpms smooths it out, but mines a Bosch VP37 distro pump unit, TDi 110.....it is in my case, I think, a small fuel leak from one of my injector pipes. Holding 1k instead of 850 idle makes the fuel pressure rise, and picks up the dragging cylinder thats getting the low pressure. Idle fuel is the equivalent of a tiny drip of diesel, around 1cc or less, so if a drip falls, thats the drip that would have got pushed thru the injector on that line.
On a common rail unit it could be a faulty injector that isnt opening with the tiny idle pulse it receives, or the rail pressure may be near the low limit. Check the difference in fuel pressure on vag com between idle and 1k rpm and see if its within spec. Should be close but if way differnt then either the pump is worn or the fuel pressure sensor may be duff.
And check for fuel leaks, even tiny ones. Also your IAT may be giving the wrong idle air temps to the ecu, but you would be able to pull a fault code for this if its open circuit or shorted, wrong values however will cause wrong fuelling, and may contribute with no code to see.