Update: the trasmission fluid was changed but i didn't resolved the problem.

I'll try to explain again what's my problem.My Car (w211 220CDI) doesn't have any problem with 1°, 2°, 3°, 4° and 5° gear at every rpm rate.But when I use my 6° gear, i get a recovery status at 3100 - 3200 rpm (180 - 185 km/h).

Any idea??