Conrad: The most expensive and frustrating method of fixing a problem is to throw parts at it.
To save money and your nerves, get a good professional diagnostic from a competent MB shop.
Fuel pressure, fuel volume, fuel flow, fuel injectors flow and spray pattern, ignition timing, ignition voltage, testing of all sensors, vaccum leaks, etc.
It may cost you a few $$. It looks like so far, your have spent quite a few $$ and you still have the same problem.
Jackd