It's definitely a bit random I think. I stopped getting nervous after my worst case of nerves which was so bad I thought about quitting, then the other week I was ready for the gig, well up for it, no pressure then stepped out onstage and found myself in need of a large whisky. Really weird, trick is styling it out I think, being rehearsed up enough that you aren't worried about the notes, then acting like you're having a great time until you settle down. Works for me and as for bum notes...that's never a reason to get worried, I'd be more worried if your groove sucked you can't really fix that, but mistakes come and go.