Did you ever try reading those (somewhat lame) self-help interview books? I found some packed away in some boxes one day. They were maybe 10 years old, but I read them anyway.
I had been having a lot of trouble with interviews before that. I realized I was doing everything wrong, and began to think of interviews in a different way. It's basically like a game. The interviewers are looking for certain qualities, and many of them will try to trip you up to get you to show bad qualities. Most of their tricks are stupid and obvious, once you know what to look for.
Now I don't have any trouble with interviews at all. I still hate them. I should mention that I'm not going after any really skilled high-paying jobs, so it may not help if you are.