Do a search for "surge," "surging," or "hesitation" along with "OM606," "E300," or "E300D." See if the symptoms described there match yours.

"Misfiring" is probably not the right word. Although you describe the engine as cold, by the time you're at 50 mph and 2200 rpm, all cylinders should be hot enough to achieve combustion even if you have a bad glow plug at startup (which would cause a brief rough idle/smoke). And, of course, there's no spark plug to not fire or fire at the wrong time. It's more likely that what you're feeling is a momentary loss of power caused by the air intake system or fuel injection system.

What surprises me is that your problem goes away when the engine is "warm." I assume you mean normal operating temp, i.e., 80-90 C.