If you don't take the job, how long will you regret that decision for? Do you think they'd have offered you the job if they shared your concerns? What do you mean that you've embarrassed yourself in the past? When this happened in the past, did you review what you would have done differently? Sounds like April and May will be the times to do things differently. How big of an audience for these presentations? Some advice... If at all possible, do a dry run in the room where the presentation will be. Don't be a tree, rooted to one spot. Move around. Use the space. Command the space.