Hello Audrius;
do you feel this switching operation at a speed of approximately 80km/h? And simultaneously to this, the engine speed is falling about some 100 rotations per minute?
Then, the solution is as easy as tranquilizing: It is the "torque converter transmission", which is activated by the transmission control unit. If this has happened and you continue accelerating, the engine speed should rise almost straight proportional to your real speed, because there is no longer any "slip" inside the gearbox.
This is also described in the "Technik"-Category of this Homepage; there is a chapter where you can find a lot of information about the gearbox.
Just follow this hyperlink:
http://www.audiv8.com/german/technology.htmlSo don't worry, there is nothing wrong with your gearbox
Hope this helps
Alex