Jiffy lube normally allows for 5 or 6 quarts of oil. Mercedes usually take 8 quarts of oil in their cars, at least everone I have owned does. So the extra quart should have been 2 extra quarts under normal conditions. Pull out the dip stick and see what you have!! Below the minimum is not good, if so then add some, This will not ruin anythign is 1500 miles. Over filled is not good. the reason the manufactures have a maximum is so that the crankshaft does not enter the oil while rotating and cause it to foam which prevents the oil form lubricating the engine correctly. I don't think that 1 quart over the max would cause a problem.
Again check the oil level.
The fluttering sounds more like a air fuel mixture problem OR as you said a spark plug problem.
Check them and let us know how they look. Color, build up, etc. Spark plugs tell alot about the burn in a cylinder.

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car