sometimes, I make a "leap" and know the right answer, and then cannot for the life of me figure out how I got there!

I do the same thing with the cryptics. Usually I will eventually manage to figure out the rationale and I normally don't write the word in until I do get to that stage.

As for those Harper's puzzles; the first one I tried to do cost me three days of puzzling just to figure out what I was supposed to do. Simple good old fashioned cryptic clues combined with instructions like:

Half of the clues are in the right place. The other half are displaced by the amount hinted at in 16A and 5D. Of those that are in the right place half are to be entered directly and the other half are anagrams.

I used to be a crossword puzzle addict; I couldn't rest at night unless I had completed at least one crossword puzzle. Then I finished a Harper's puzzle and the addiction was broken.