I don't have a very severe stutter. It was more pronounced when I was younger, but it's still there hiding, I block on certain words sometimes. I recognise what you mean with it being worse from time to time. Same for me, I can be completely fluent but some days or weeks I really have to focus to not get stuck or stutter. I don't really have many tips. Maybe try to slow down your speech if that helps, but sometimes it does not, I know. Maybe try asking some questions to the interviewer to avoid having the entire interview revolve around yourself. Maybe this could make you feel less under pressure.