You know I kind of get that with people that are really hot, or people that are super boring I just can't act interested.
I am like this too. I get super nervous some of the time, but I always stagger my words and mix up my sentences and then try to fix them, and as that is happening I feel myself turning red, and they are just sitting there with a "I feel like I'm talking to a little kid" look on their face.
Maybe try and talk about some of the things you admire about them. Like if you admire what they do for a living talk about that.
I often dream that I meet some superstar (Mick Jagger or Snoop Dogg) and I act stupid and cant stop gushing... Ugh, I'd hate that.