With all the preparation and practice you're in a good position to do well on the day. Try taking deep slow breaths to relax yourself before the interview. Organisations such as the Civil Service often use a panel to interview candidates. Try not to aim your answers exclusively at one interviewer- include all the panellists in your answers and eye contact. Tailoring your answers to fit a diverse panel is a difficult skill to master.
Personal clashes are much more significant in one to one interviews.
Another possibility is facing an interviewer who deliberately attempts to place you under stress. They may use an aggressive questioning technique, or ask very tough questions. This is designed to see how you cope with pressure. Remain calm and answer each question rationally. If possible find out before the interview what format it will be taking.