Cold start -> OK
Hot Start -> OK

Warm start -> problematic.

I agree with one previous post. This is a typical symptom of slightly leaky fuel injectors.

When they leak a little, hot start is not affected because there is not enough fuel to leak out. Cold start is also not affected because there is enough time for the leaked fuel to evaporate.

Warm start is affected because there is leaked fuel (flood). Enough fuel has leaked and not enough time for it to evaporate. You can further confirm by doing a double start. Start it (it may not start). Stop and wait for a few seconds. Start it again (it should start now). The first (failed) start will burn off the leaked fuel and the second start will be successful.

Before you try to replace the injectors, try to clean using an approved fuel injector cleaner. Contrary to what have said by many, MB does approve two injector cleaners in their 2006 approved service products for all engines. On page 15, one is an MB product that you can buy from the dealer. Another is Chevron Techron.

You can get a 20 oz bottle and put in the tank and filled it up. Do a mixed driving and preferably over a few days. Let the car sit after driving (Let it leak so the cleaner can clean the injectors to simulate the leaky situation). Or you can go to Advance Auto Parts and get two 12 oz bottle (buy one get one free before the end of this month).

You should notice some improvement after one application (a few successful warm starts). You may need a second application. Change the oil after the applications.

This should cure deposit related leaks. If it is an internal (parts related), then this cleaning will not fix.