You are really wasting your money on blind repairs.
Find a real OBDII specialist who really understands the readings from the OBDII, the CAN bus, and the various vehicle controllers. They normally use good trouble shooting OBDII/CAN computers/interfaces, analyzers, and other testing equipment. Mercedes is no different than any other electronic based vehicle.
Any experienced/educated mechanic that understands vehicle electronics, will use a multi-channel 4/2 channel oscilloscope together with other specialized hardware, to gather readings (pressure, ignition readings, timing, firing patterns, VE, sensor signal shapes, etc) which can be used to pinpoint if it is a sensor (MAF, CPS, etc) coils, controller, etc. They use multiple readings from key parts of the engine electronics, exhaust, fuel pressure readings, signals from the Maf, etc. to understand the exact state of the engine and where the problem is coming from. Each electronic/electrical problem has a specific signal/amplitude signature and together with the other measurements: fuel pressure, exhaust readings, etc. is enough data for an experienced mechanic to find where is the problem or to discard the elements that are working fine: MAF, CPS, etc.
Mercedes Benz mechanics seem just trained to blindly change parts and not to understand the vehicle electronics and identify the failing part. It seems that very few of them are trained enough to debug problematic vehicle electronics and most of them seem to choose the more expensive (profitable for the dealer) and less time consumming workflow. They will change a transmission, when only the transmission controller or a solenoid is bad, or will change O2 sensors, catalytic converters, when only the MAF sensor is dirty or failing.
The link below has a nice article discussing various OBDII/CAN scanners:
http://www.x431usa.com/Roundel%20Cracking-The-Code-May2006.pdf
Though, my blondie wife says you need to get another "M" whatever that may mean..
:crybaby2:
AC