I have the same question about when I'm around certain people. Normally, I can speak fluent english/dutch without any problems. Whenever I'm near a certain someone, I keep saying shit that doesn't make sense, keep making grammatical mistakes, etc. Always makes me look like some idiot.
Its a confrontation problem, certain people you feel more comfortable with, make you relax more while talking. Think of relatives, friends, girlfriend etc. When you confront someone you dont know much/fear a little. Like your boss for instance, certain people have the need to show those people that they are confident in their speaking. Sadly, trying hard makes you stressed and not relaxed which makes these mistakes. I Luckily never had the problem and can speak quite relaxed with everyone.
Actually, I tend to make more mistakes near people I'm comfortable with, because I know mistakes will matter less there. But sometimes there are those friends who still make a big point about every single mistake xd
Oh, Yeah if you make mistakes near comfortable people its probably the fact to speak 'lazy' as they call it. But that has no problem with your conversations with them since you sort of do it on purpose.
I speak rather eloquent english/spanish, but one of my problems is... if I get distracted, my words will get jumbled up, or a letter switches, or I simply replace a word with something else. x) You'd be surprised how much effort talking takes.