Jeri, Carol and Wolfgang (IMO) have touched on the essence of what I learned long ago when trying to memorize long, intricate piano pieces. One year I totally blanked out on a Rachmaninoff piece (my finger memory failed me under stress), so my teacher made me memorize the next piece (a long Brahms) in two different ways. 1) finger memory 2) cognitive memory Jon, I think the answer (and solution) to your question is somewhere in these two ways of learning to play music.

Wolfgang describes finger memory as motor memory. I always thought of it as right brain. Now I think it must be both. Anyway, it's the autopilot you speak of, Jon. Much like touch typing, practice over and over to instill this in your brain. It helps to practice very slowly with an enforced metronome beat and gradually increase the speed. Sing along with the tune in your brain. Also, use accents, exagerated motor movements, anything to make your fingers play the passage again. And practice the difficult passage at least ten times to every one time you play the whole song.

The other method, cognitive learning, involves the left side of the brain and being able to verbalize what is happening in the music. I would have to be able to start the piece at any point every four measures. It's amazing how many times we have to start at the beginning of a piece whenever we only learn it by finger memory. Also, analyze the actual note names, chords, patterns, etc.

As far as mixing tunes...I dunno. I wonder if there is some kind of transfer here. I never had that problem. Actually I found learning patterns extremely helpful for sight reading.