Oh poor you funkychicken, I can really empathise. I'm very good at making presentations and talking to new people, except in interviews. Somehow it feels so personal, I lose my confidence. But I always pause, take a deep breath, and if it's really bad I also tell them I'm nervous. Nobody has ever had a problem with me saying that, and I have been offered jobs even after what I thought were bad stuttery interviews.
I'm sure many interviewers would rather hire someone who obviously cares about the job and getting things right than someone who walks in like they're already in charge. The person hiring has been in your shoes and knows what it feels like.
If you feel like you might sound over-rehearsed, have you considered taking a more laid-back approach, and just having an overview of what you want to say/ask instead of memorising potential answers? I always find it's better to listen to their questions and answer in a focused way rather than trying to crowbar in stock answers you think are good. My technique, if they ask me a tricky one, is to bat it back to them and ask, 'in what way?', or 'what do you mean?' then while they are rephrasing the question it gives me longer to think of a more pithy answer!
My worst interview was during a bout of food poisoning and I actually vommed in the middle of a question. It was a phone interview and I had the presence of mind to cut them off. When they called back I blamed the signal. They offered me the job. You can't get much worse than that! 
Good luck, really wishing you the best with your job hunt!