We've all been there/are there. Seeing that you're usually fluent, i guess it has to do with anxiety when talking to your higher ups. I'd focus on this anxiety and what causes it. As for any form of anxiety: exposure is the best therapy. Be confident and talk to them anyway. Say whatever you actually want to say inside, no matter how long it takes. They'll respect that I'm sure. If they don't, find another job. You can heal anxiety and this will heal stutering. Not the other way around.