Could be the tool is loose in the collet and getting pulled into the job. Been there done that although often breaks the tool.

Could be the gcode is calling for Z drops for some reason so manually check the code (or post here if not too long)

Could be electrical with loose wire or interference. Do an air cut away from the part without the spindle running and watch what it does. Ask if you don’t know how to do that.

For etiquette usually a fresh post is required although the original requestor has not replied so maybe carry on !