Same thing happens to me. When I'm comfortable, like when I'm chatting online or with a close friend, I'm quick thinking, intelligent, and can speak properly. But if I'm in a group or talking to a person I don't know, my mind freezes up.
I forget people's names, place names, the right words. Examples:
"What you just said about Paris reminds me of... of when I was in the city of... well, its this city in Germany, like, in the south, umm.. I forget the name now, but I stayed there for a month and it was like Paris."
"Remember when you dated that girl.... umm, what's her name, and I was her math tutor? You know, her name was like.... well... you must know, you dated her!..."
"I couldn't turn my car. It was so hard... turning the wheels, you know, holding on circling it right but going left." (When I meant to simply say "It was hard to steer," with 'steer' being the proper word.)
Some of the solutions mentioned here sound good. I'm going to try them. Good luck.