I had this happen to me twice, the first time was with my 1986 300E that I had the fluid changed in, shortly afterwards, it began to act up and finally would randomly shift out of gear all the time and it came to the point that the transmission was gone and needed to be replaced.

The second time was in my 91 300E that I currently have, I had the fluid changed which I was very very weary about doing because of the experience mentioned above. Fluid was changed and all seemed well until the exact same thing happened to me and I was really bummed, talked to the mechanic and he told me to wait it out, that the transmission just needed to "adjust" to the thinner clean fluid and it needed to break in. So I drove it, with occasional acting ups it would do until finally it stopped acting up and starting operating normally. To this day, I haven't had any problems and it shifts normally.

Hopefully your transmission just needs to adjust to the fluid as mine had to, but I mean if it keeps doing it for a really long time, it might be time to have work done to the transmission, which sucks. Hopefully it's the "break in" period!

__________________
76 MG Midget
78 Datsun 280Z
86 Peugeot 505
01 Subaru Outback

“Owning a car is like owning a dog. Every now and then, it craps on the carpet — but it’s still your dog.” - Jeremy Clarkson