They all do that when they age,I have not seen a smooth running 380SEL/380SEC in 20 years,the "computer" needs to be resoldered,vacuum leaks,the factory spec copper spark plugs,Made in Germany are nowhere to be found in the last 10 years,the ones that Bosch makes in India are not 2 alike,terrible,so,one cannot even tune the car properly,the platinum plugs are a no no for this car...then it runs rich,black smoke,and so on,get use to it,and if you get better then 12 mpg,it runs fine.