It's possible, like others said earlier, that your head gasket could be bad (Mercedes is notorious for defective head gaskets), and if so, you should be able to check with a CO coolant leak checker. When you shut down your diesel, the coolant is around 20psi, and is around this pressure for quite a while; if head gasket is bad, your coolant could be ever so slowly leaking into one of your cylinders, enough to cause what your symptoms are. You should have a bit of extra pressure build-up when you open the radiator cap on a COLD engine after about 5 minutes warm-up, but not necessarily so. Also, it would clean itself out fairly quickly; too much drainage will create hydraulic engine lock-up, and you will know when this happens

I would tend to lean towards the fuel-related dead cylinder(s) diagnosis that the others suggest, however, given the time duration for warm-up that you speak of. Could also be low compression in one or more cylinders.

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