Why do I sometimes do dumb things?

Sometimes I'll just randomly do very dumb things. It really only happens around my friends. I guess I get nervous sometimes, like if I'm driving them to a restaurant, I panic that I'll get lost and look like and idiot. I have to concentrate on where I'm going, and might make wrong turns or miss an exit, whereas when I'm by myself I could drive there without thinking. I'm not a stupid person, but sometimes I feel that way. I'm starting to think I have some social anxiety disorder. Even when I'm perfectly calm and not anxious, I find that I do dumb things more frequently when I'm with other people, like being clumsy and dropping things. I guess my friends find me entertaining, but personally I feel embared. And what's worse, drinking amplifies my clumsiness and sometimes makes me spacey. Otherwise I'm a very smart guy, highly educated with demanding job. I just wonder what's with the inconsistency?