I resolved the extruder problem on an radical way: shorten the ptfe-tube and change the extruder place. So I did not longer here thet snap over while printing perimeters with 30mm/s. The second step was to reduce the speed of filling from 60mm/s@210° to 50mm/s@210° for PLA. Now it works better, but not perfekt. I ordered an geared stepper. lets see if that will help.