DS 250 focus issue

I’m experiencing some dificulty with my moving light.

Design spot 250

The problem affect the focus of the moving light. The focus is alway’s bad at dmx 128. To obtain a good focus I need to put the dmx at 255.
When I reset all motor(on display panel) the moving light focus become good at dmx 200, but only for a short periode of time.

Is ther any ajustment I can do myself ?

Thanks for help.