The part is the center portion I take it? If so, you may want to leave holding tabs on the flat faces to keep it from vibrating and getting ruined as it drops away. Hopefully you are using software that has that feature. It's a bear to have to add them manually. Also I usually try to open the slot to 150% of the cutter width as I go to keep from have a chip-binding mess in the slot. Depends on what you software allows but say it's a 1/4" slot I'll specify a 3/8" cut-width so it opens that trench up as it works it's way down. I find it's better to have a sizable final cut depth too. Don't leave a tin-foil layer for the last pass, having a 1/16" or more to cut into in the last means the material helps keep it stable and I get less chatter till the very end.
Holding tabs though.... they really help but they mean some manual finishing work is needed after you break it loose.