I'm not really performance-shy when it comes to music (at least, it gives me something to focus on, like my bandmates' cues), but I hate being alone in front of a group in class. In both cases, without warning (even if I felt fairly confident) I feel myself starting to shake uncontrollably. When I am performing music, it goes away, but when I am supposed to talk, I am so focussed on the people around me that I can't help myself. The pressure I put on myself is just too high to cope at that moment. It only becomes worse if I manage to embarass myself, which is only each time I have to do a presentation.
About specifically tests, it's only the time pressure that gets to me. If I know the stuff, I could do it blindfolded (figuratively, of course). But if it's something general, like an IQ test, with logic questions, like those where you have to decide which (cube, number, anything) is next, I get so nervous and fidgety that I can really freak out. It's like I know that if I weren't so nervous I could do it considerably faster, but at that moment, my mind goes completely blank. Walking and distracting isn't really applicable in this case.
Well, coming back to music... I don't know... it's just sort of my thing. I'm good at singing (I felt only half-insulted when they referred to me as "radio" in fourth grade because I knew I was good; it also helped that a lot of people told me so), I am confident about it. It is pretty much the only thing I am positively confident about. Maybe this is why it is the exception.