I know exactly what you mean. I was there for the first two months myself. Are you rushing through the questions, timing yourself? That was what had me really messing up. I kept thinking that on the real thing I would only have 2 mins per question and that pressure made you mess up even more. My best advice is to take your time and work through the OG completely and do not time yourself. You may take more practice exams when you are doing this, but your best option would be to keep working through the book. Go over your answers methodically and see which ones gave your problems. With time, your silly mistake problems will subside, but remember you are human and you may make some mistakes sometimes. Just keep a positive attitude, breathe deep and try again. DO NOT GIVE UP HOPE. You will do well, just be confident and keep trying. Have you used the score estimator? Often, these practice exams can be harder than the real thing (take this from someone who has taken the real thing twice).
As for the RC, do you formulate your own answers before you read the answer choices available?Take it from an English teacher, that helps alot. Out of scope questions usually is a problem for the more intelligent. Approach each question (I learned this from GMAX online-you can read the review on the website) like you are an alien and the only information you have about the topic of the essay is what you see in front of you. With the information that is in the passage (only what is in the passage) what can you conclude? Create your own answer and then look for the closest answer to yours. It is difficult at first, but if you practice it will become easier. Keep positive!!
And whatsoever ye do do it as unto the Lord and not unto man