Has your Mechanic done a fuel pressure test? It's a fairly well know proceedure to anyone that works on CIS or CIS-E fuel systems. It sounds like a fuel pressure problem.
If the pressure test shows no loss of fuel pressure,the injectors are the next logical step. You need to find someone that will test the injectors for leakage and spray pattern. Usually the failure is only evident at very low delivery volume. If the injectors were sent out for cleaning, chances are they were just flushed-out. If they test bad replacement is the only "fix".
ASE Master Tech
Mercedes Tech 19 Years