Well, I´m sitting the first set of JARs in April, and I will confirm that instruments is an absolute bitch. The problem is not so much understanding the subject, but rather trying to figure out the correct answer to the question. Remember the paper is set by the French, and in some areas they have a very different understanding from the rest of us ! For some questions there can be more than one correct answer, or no answer which is totally correct. It is even at the stage where the schools are not 100% sure what the questions are asking.
Part of the problem seems to be that there is no generic aircraft for the instruments paper whereas with Nav Gen all questions relate to the 737-400. Questions is instruments may refer to ´An autopilot´, but there are so many different types and designs that it is often possible to find references which will produce different answers.
So, my advice on this would be if you can get hold of any French notes and get a frog to translate them you may stand a chance of having a rough idea what is going on
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CW