The problem's not even that I'm bad under pressure, and in fact I've repeatedly been told the exact opposite by people who've worked with me. The problem is specifically about doing a programming performance in front of an audience, in an interview rather than co-worker context. I'm god-awful at that unless I'm heavily prepped for exactly what's going on. No tools? I'll forget basic syntax, yes even in "a language I chose". Let me use tools? I'll forget how to use them, or get self-conscious and avoid things I'm not entirely sure will work, overriding my own muscle memory. It's a very specific problem but I doubt I'm the only one who absolutely can do the job just fine, but is also entirely capable of coming off like a complete "faker" in an interview.
[EDIT] to make matters worse I can talk about programming just fine in that setting, which probably adds to the "he's some kind of social engineering genius who learned to sound exactly like a competent programmer while somehow also not learning a single thing about programming" impression. It's not usually an issue, but I'm quite sure I've reinforced some interviewers' notion that it's a good thing they do whiteboard screening because they're overwhelmed with lying applicants, and that I was one of them.