Kitamura Mycenter 0 ATC problem

I got Kitamura Mycenter Zero (-89). Atc arm have some problem, it moves too slow and machine alarms tool changing taking too much time.

What is strange here, is atc arm move fast first's changes but it getting slower every tool change.

I found Mitsubishi frequency converter in electrical cabinet and it is connected to atc arm motor. Have there frequency converter originally or is this some kind of DIY-fix?

Probally problem is in that VFD, but I like to make sure is there known mechanical problem.

I hope some one have answer for this...