Problem seems solved, thank you all :)
rebuild it completely and discovered that in one pulley the tightening screw was not completely tight! just half a turn so the pulley looked like following the motor rotation perfectly but it slipped just a little bit when the motor changed direction. also couldnt see this from outside because the pulley covered the motor axis completely. been looking desperately for the problem for more than 2 months and now its just that little creepy screw :X
thank you all again for the help an all the suggestions !
rebuild it completely and discovered that in one pulley the tightening screw was not completely tight! just half a turn so the pulley looked like following the motor rotation perfectly but it slipped just a little bit when the motor changed direction. also couldnt see this from outside because the pulley covered the motor axis completely. been looking desperately for the problem for more than 2 months and now its just that little creepy screw :X
thank you all again for the help an all the suggestions !