Cutting Extruded PS

Wow. Extruded polystyrene is much harder to cut properly than expanded. I just went thru a whole board and can't find a setting where I cut clean without having almost 4 millimeters of cut width. That just won't do. I'm going to ask a stupid question on the CNC mailing list and see what the response is. To whit : Should I be trying to cut at very slow speeds? Under 1mm/sec? Would that allow a lower heat setting, and a narrow cut? Location : Beauvais, France