Why do I get so shy and awkward around some people but not others?

Okay, so I am not completely socially awkward. I have a group of friends that I talk to and act goofy around all the time and have no problem. However, when I get around this other group of people, I suddenly get realy quiet and have nothing to say. I feel bad because I get the feeling that they are getting this impression of me being snooty, but I really have no idea what to say when I'm around them. I don't know if it's because they intimidate me or because they're different than me, maybe I have confidence issues, but for some reason I become really shy around them. I know the answer seems kind of obvious- speak up and be myself, but I just don't for some reason. How do I fix this?