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.