I mean whenever I know I am correct, I am pretty much confident with the Panel who is interviewing me.But when I am not sure about certain topic I tend to murmur or get nervous and that one thing increases my nervousness to a greater extent and baam Whatever Correct Stuff i know is overshadowed by this nervousness....I mean how to control it or get rid of it....Is it ok if we say "I dont know"
Once we had a big escalation because somebody said "he doesnot know, he will check it up and update him/her"
2. Be yourself. No one knows everything.
3. Treat each interview as a free training course. This will give you a win/win situation. If you get the job, you win. If you don't get the job, you will be in a position to know the areas you need to concentrate more to do better next time.
Nobody knows everything. I look for logical thinking ability, in case a the candidate I am interviewing does not know about some topics. That itself tells me if the candidate is promising or not.
It is natural to get nervous. The trick is in controlling the nervousness. If the interviewer is good he will immediately zone into your nervousness. After all he would also like to evaluate how you perform under pressure.
Overall, like Tim said, the best approach is "This is a gray area for me, but logically it should be...."
When you are correct, I don't see any issue (and as you've mentioned, you can convince interview panel).
But if you don't know the answer, do not ever fake it. Either say I don't know (right away), or if applicable, you can ask further clarification about question, and state what kind of knowledge you have in that topic.
During a programming puzzle etc., do not mumble/murmur. It either shows that you are not confident, or you are making up the stuff. If you are thinking about the answer (i.e. programming question), think aloud. Let the interviewer know what you are thinking and what logical approach you are following. If you keep on staring the paper/screen even for 30 seconds, interviewer starts to get annoyed (because he/she have no idea what's going on in your mind). Do paperwork if needed. This way, even if there's a small glitch/bug in your code/logic, it would be easier to identify and fix it. Never run in hurry to solve a problem. Nobody wants an employee who can give wrong answer in 2 seconds.
Further, if you have any habits like scratching your nose, wiping off your forehead with your fingers etc., then give them up. People do lot of weird stuff (like tapping the pencil on table etc.) when they are deep in thoughts (an in my personal opinion, nothing is wrong in that), but interview is not the place to show off that stuff. There, you have to be presentable (not just clothes etc., but also with body language). So be patient, be calm and approach the problem with step by step. Also, either ask or tell the assumptions you've made at the beginning itself.
Finally, as some of the replies have already mentioned, nobody knows everything, so if you don't know anything, tell it politely and look into the eyes of interviewer while you do so.