Engine Hiccup

At last I got the transmission oil and filter changed by a professional mechanic about 4 weeks ago. When I got it back everything was perfect, driveability that is. But gradually the same problem has been coming back but at much milder intensity. After a long trip, gear shifting is just not perfect at slow downs and starting offs. At start offs there is a little hesitation. I don't know whether I'm being too picky or not. I have an ML350 and it runs very smoothly under all conditions. I checked the spark plugs they look fine, no burns, oil, or damage at the tips. Mind you they haven't been changed for the past 7 years (approx 80k miles). I don't know if the last owner ever changed them either.
Your thoughts please.


