P0170,P0173,P0300 99 C280

I have a 99 C280 with 94000 miles, and I had the fuel trim malfunction bank 1&2 codes and now with P0300, multiple cylinder misfire and P0304-6, drivers side. I have had a schedule B service, spark plugs,
fuel filter, air filter, etc. and took the car to a mechanic to diagnose the fuel trim malfunction. Replaced what he said was a faulty O2 sensor, paid a bunch of money but code came back within a couple of weeks. He also checked the MAF, said that values were a little low but within range. I also cleaned the MAF with electrical contact cleaner. No noticeable improvement. Bought fuel system cleaner but no change in performance and no light out. There has been a drop in performance, about 20%, but MPG is 27/28. Occasionally, with the fuel trim error, the car surges to near red-line RPM under acceleration for one second and drops down. The cylinder misfires occured last week after a cold start, bucked and jolted roughly, but after a while I was able to take it home fine. Havent used the car since. On startup, exhaust has slight fuel odor, similar to my sister's 2005 grand caravan, but no exhaust smoke or odors after warm. Car ran ok until the misfires. Anyone have any advice on how to approach this problem? What is typically looked at, and can I do any of it before taking it to a mechanic?Also, I will like to know what people prefer, a dealer or good independent mechanic? I have the StarTek manual and can borrow a couple of tools from family members.
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Our 2001 E320 had the same code. Inexperienced mechanic would assume O2 sensors, but this is not the case. The fix was a new MAF sensor.
Only the dealer is equipped to diagnose the problem as they have proprietary diagnostic machine to thoroughly test the system. Other mechanic can only guess. Yes, O2 sensor can cause the problem, but O2 sensor can also be tested wtih a simple multimeter to see if the value is within spec just like that mechanic tested your MAF.
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