i`ve got this problem with two autoreverse walkmen, but it is always when playing B side, like shown in the video...  A is ok o.O ... and it seems, that its coming worse with light and bright sounds, i cannot hear it playing songs with more bass instruments and boom... so what is it ? i changed a lot belts, but no effect -.-... 

Sounds an awful lot like misalignment of the tape head when it's in "B-side mode". You can try to adjust the azimuth, but with auto reverse decks, it's quite difficult to get it just right.

mmh good idea.. but screwing around on the head just gave me more or less damped sound ... but not this .. vibrated... jerky .... does anybody else have those problems ?

     Yes and it's normally the rubber pinch roller that is out of round even though it seems Round - will give you that jerky sound on the 'b'-side as you normally dont play that side too much and that other 'b'-side rubber pinch roller hasn't gotten much exercise as the a-side pinch roller rubber wheel has

     Look to your Rubber pinch roller's after all other troubleshooting ( belt's replaced, new batteries) have failed to remedy your sound playback 'jerkiness'.

     Try applying rubber rejuvenator to your pinch roller's A & B side and see how well that help's, short of replacing your pinch roller's which are really hard to find...

but i am wondering about, if its the roller ... why the deeper sounding tracks are less (not ?) affected ? ...

Like what ElektroNeko said about the azimuth & I'll add that your Tape head may be worn more on One side than the other giving you his issue. Also have you tried de-magnetizing your tape-head after all this I am at a loss as to what may be wrong with your unit other than to suggest that there may be issue's in the electronic's of the handling of the 'head' by your equipment that may be faulty, of which is beyound my ability to repair. ( You may need to send it to Retropcdos for service ) would be best

mmh ok.. cleaned the roller, as far as possible.... it doesn`t really help ... cannot say anything about the heads.. but one device are sounding very damped, and is impossible to justage it, that BOTH sides are sounding well..

cannot demagnetize ... looking for a demag cassette from TDK, but silly prices ...

atm, i`ve got problems with a tapedeck too.. yes, its an autoreverse deck, and yes .. its the b side again ... +argh+ .. hate this ...