You know, this is funny, because vbraun also showed me this link last night. Perhaps he decided to show you too....it's a very interesting article, and for the most part, can be related to nexuiz. I do in fact find myself "choking" when I get frustrated during a game from several causes. If I miss like 3 nex shots in a row, for example, I try to actually concentrate on hitting the next one, even though normally I wouldn't need to. Chances are, I would miss it. Why? Once, if I remember correctly, I remembered that clearing your mind before you do anything, whether it be sports, gaming, or even just a test, would greatly benefit your performance. I think it's because that when you're thinking of things like aiming, it takes more time to process in your mind that you would normally do without thinking..perhaps causing delay? I dunno if this is exactly true, but it would make sense if it does..
I'm the kind of person who expects too much from himself. Because of this, I can easily frustrate myself, and ultimately take my concentration away from the game, giving away frags, weapons, and megas, and sometimes, get lost. Lost? I don't mean lost physically on the map, I mean lost as in you don't know what to do next. From here, this can even go so far as to help you become panicked, which would, technically, be a total different playing style, but you still won't play fluidly.
Some people, like panzer, get nervous when they play some of the better duelers. You can consider this panicking...because when you're panicking, as described in the article, you rely on instinct and second nature entirely rather than actually thinking. Now, this may not always be a bad thing, but it takes your concentration off of things like mega timings, weapon collection, and map routes, and has more of a focus on things like aim and moving around.
Of course, you can avoid choking or panicking by practice, which gives you experience. However, you must remind yourself that if you're not willing to go the extra mile, put in all the effort you can, it will take a much longer time to really get good at it. Now I'm not saying I do it all the time myself...I'll admit I still get frustrated when I'm playing sometimes, but I'm working on that.