If the machine is mechanically sound (always check per: Machine operating checklist ) then other considerations are:
You have your file set up to cut slots just as narrow as the endmill is wide — this results in near constant 100% tooling engagement which is a difficult cut for the machine to hold — best working practice is to add geometry and cut as a pocket down to tab depth, or the penultimate pass.
Other possible considerations are tooling and feeds and speeds and workholding — check them as well.