Grbl freeze problem

I have a 10w optical output diode laser running with a original Arduino & Grbl. I habe the problem, that while engraving, the laser sometimes freezes with the diode either on or off, butt the gantry is still standing. It does not do that while cutting. I have tried switching from buffered streaming to continuous and back, but it didnt help. Different PSU / usb cable also didnt. Any ideas why?