Hi jmcnyc,
Please don't be discouraged. . . although I'm sure certain schools are exceptions, in general I don't believe that med schools hold stress interviews in high regard anymore. I've had six interviews, but considering some of them were with two interviewers it adds up to nine independent interview sessions. . . and although I DID have one doozy of a stress interview, all of the others were purely conversational, to the point where I *enjoyed* most of them! And even with my stress interview, it appears that this was an unusual case, as the school at which I interviewed is known for giving relaxed and comfortable sessions.
It's very unfortunate that your first interview was a negative experience; very unsettling, and just bad luck that it had to come first, as I know how anxious it must make you feel! But please believe that yours was not altogether a common example. It will improve; don't give up! Just have faith in who you are and what you know you can do, and you'll shine through with no problem at all.
If it is any consolation, also. . . I was nearly 99.9% positive that I would be rejected from the school at which I had my awful interview, but I received an acceptance letter last week.
Best of luck to you!