ignazzio

This is all about 1998 A6 2.8 quattro, which sometimes bucks, shudders just for a second or two,only during drive, when rpm drops below 1800. But it does not happen every time the rpm goes below 1800. It feels like a short vibration...knock,knock...but accelerating makes it's disappear for that moment. Today I have checked this car with vag-com and it found no engine fault codes. Three months ago when it first started buckling I changed the spark plugs, this shudder/buck vanished, couple weeks ago it started again. I checked the spark plugs yesterday, they had a little bit of oil on white ceramic part (insulator),metal shell (hex) and gasket,but I believe oil came because of 'poor' valve cover gasket,which I am planning to change in nearly future, but I don't think it has any impact on the vibration itself, because terminal nut, thread and firing end were dry.
Does anyone know what the problem might be, when computer finds no fault codes?