Sorry to write in English. お返事日本語でも大丈夫です！
I have a 3.5 model.
I have no problems cutting small shapes. I use 300speed 100%power and 6 passes with 2.5mm MDF.
On larger shapes and especially straight lines I get these wavy burn lines. The board is stuck down with strong magnets so the MDF is not warped. The belts look OK and the laser head is clean. Do you have any ideas what could be causing this?