auto transmission fluid

Hi I’m a newbie, I have a cls 2007 and have auto problems even though I’ve just had the oil and filter changed. It started when I noticed that I was loosing kick down so had oil and filter changed. Sometimes I still loose kick down. If I stop and turn engine off it seems to reset it and I have...

E320 CDi not changing gear properly, and car not moving beyond 40 mph or 3000 revs. I have recently bought the car and the previous owner says he never changed the transmission fluid or filter during previous 7 years. On checking, the transmission fluid is black. Could this be causing the above...