Hi
Hard to tell from your description, but it sounds like one or more cylinders are slow to start firing at cranking speed. The minimum cranking speed for the fuel/air to get hot enough to self-ignite depends on the temperature (with working glow plugs if required) and the compression. So to start a slightly worn engine on a cold day it may need to be turned over a bit faster than your battery/wiring/starter motor is presently capable of. Whilst the starter pinion is engaged, the engine can't turn over any faster than cranking speed, so once you release the key it's possible to coax the revs up using the cylinders that are firing and get the others to join in. The shaking and lumpiness will then smooth out. You may see some white smoke initially which is unburnt diesel being ejected from the non-firing cylinders.
Does the engine crank reasonably vigorously ? If not, you may need to check the battery and the heavy wiring from battery to starter and back via the engine block and the block to body earth strap. These are big engines and the starters draw hundreds of amps so the battery and all the wiring joints need to be tip-top.
